Why am I getting so much worse?

Probably because you are looking for tactics when they are not there. You are likely neglecting basic strategic principles in order to force long fancy ways of exchanging pieces which don't actually benefit you.

It's the same with me. I started at 800 - advanced to 1100 and then I have a long losing streak and go backwards. I think maybe sometimes I'm too aggressive and daring - and cocky - only to get slaughtered. I also tend to lose far too many games on time - even if I'm a queen up I still end up losing. Maybe I don't keep a sharp eye on the clock. I'll try and change that.
The thing is, I know the tactics are there. At my level people make mistakes all the time. If analyze the game afterwards I invaribly find that there were many blunders and mistakes on both sides. At my level, there are defintely tactics all over the place.

Is this about playing over the Net or face to face chess? There are different psychological & physiological processes in each which could affect performance. My hearbeat remains stable when I play online and I don't drink so much water :)

Probably because you are looking for tactics when they are not there. You are likely neglecting basic strategic principles in order to force long fancy ways of exchanging pieces which don't actually benefit you.
I agree with this, super true..:$

You are correct in that there are tactics everywhere however I don't believe that this is just cause to always be looking for them. The reason that you blunder all the time is because you aren't good enough to see the tactics that are always there. Therefore it is easier to think of them as not being there.
A computer may see a 7 move combination that wins a Queen but you are never going to see it so why look for it? Simply think of it as not being there. If you miss a simple fork or a mate in two then you have blundered so look for them. Don't look for what might be there when you will never find it even when it is.

I looked at some of your games. They are odd. In the first two you dropped a piece and promptly resigned, the first time the piece had got into an awkward position and your oponent invested all of two moves in trapping it, the second time you threw the piece away capturing a guarded pawn. The next two games are particularly odd. In the first you resigned after a dozen or so moves in a game where material was equal and nothing much had yet happened; then in the second you resigned after a similar number of moves at a point where your opponent had just put his or her queen en pris.
Tactics training has nothing to do with any of these lost games. Short attention span perhaps. Or playing when you don't have enough time to finish a game.
Try some on line games with a three day time limit.

I also looked at a handful of your games, and I noticed some of the same things that johnyoudell did. However, I think it's a combination of two factors; 1) Sometimes not counting the pieces before going into an exchange, resulting in losing material (which is really a basic tactical mistake), and 2) Playing hope chess instead of real chess (to use a Heismanism). Basically, hope chess is failing to look at all checks, captures, and threats for your moves and your opponent's moves.
I'd work on those two issues if I were you.

That isn't what hope chess is. All you described is bad chess. Hope chess is setting lots of avoidable traps that don't help your position unless your opponent misses the trap.

That isn't what hope chess is. All you described is bad chess. Hope chess is setting lots of avoidable traps that don't help your position unless your opponent misses the trap.
The term "hope chess" coined by Heisman, is playing a move and hoping there isn't a move that immediately refutes it. Or in other words the player isn't looking at all the forcing moves his opponent has available. He says (and I'm sure he's right) this is a very common mistake happening even in 1500 USCF games (where you may think these players are beyond that).

I looked at some of your games. They are odd. In the first two you dropped a piece and promptly resigned, the first time the piece had got into an awkward position and your oponent invested all of two moves in trapping it, the second time you threw the piece away capturing a guarded pawn. The next two games are particularly odd. In the first you resigned after a dozen or so moves in a game where material was equal and nothing much had yet happened; then in the second you resigned after a similar number of moves at a point where your opponent had just put his or her queen en pris.
Tactics training has nothing to do with any of these lost games. Short attention span perhaps. Or playing when you don't have enough time to finish a game.
Try some on line games with a three day time limit.
Sounds like sandbagging.

I have practicing a lot of tactics on chess tempo, but it seems the more tactics I practice and the better I get on chesstempo, the worse my playing gets. I've just can't seem to win any games against anyone who's 1200 or higher anymore. It seems that no matter what I do, they have always calculated one move further and what I thought was a great tactical idea turns out to be a losing move.
Sounds like you're doing blitz puzzles and just guessing at the solution instead of calculating a win and after practicing this guess method you start guessing in your own games resulting in more losses (makes sense to me).

- Play longer games. Take all your time. Make sure that when you execute tactics, that you see them clearly. Like EscherehcsE said, fully analyze exchanges and examine all checks, captures and threats.
- Perhaps you need to take a break from tactics and concentrate on playing actual games for a while. There are certain skills that will only come from actually playing chess.

The thing is, I know the tactics are there. At my level people make mistakes all the time. If analyze the game afterwards I invaribly find that there were many blunders and mistakes on both sides. At my level, there are defintely tactics all over the place.
If I open a game with a3, c3, e3, g3 against a beginner I'll eventually get a winning tactic if I wait long enough... but if I continue the goofy moves maybe not!
As Fischer said tactics flow from a superior position. If you're just waiting for them to appear for you without making good positional moves it may be a very long wait even against weak players.