as i see it from here you suspect someone of cheating. analize the online games with an engine to see if the moves correspond in a high percentage... after that report the person so you will reconcile yourself finding, officially, that he cheated or not..
2100 online - 1400 blitz?
A couple things:
1) In regards to your original post, did you actually calculate his average opponent rating? The one listed by chess.com is only for the last 90 days. If his overall average opponent rating was 2136 and he was +46-9=10, he'd be rated a lot higher than 2100. chess.com says my average opponent rating is 2211, which is a complete lie, since it's only based on my past 3 (out of 59) opponents. My real average opponent rating is probably around 1800.
2) Everyone is different, with different strengths and weaknesses. I personally suck at openings, so I often find myself at a disadvantage in blitz chess. But in online chess games my better endgame play becomes an asset, and I'm able to avoid most opening problems with the help of the Game Explorer. So my online rating is almost 500 points more than my blitz rating.
You could have extremes in either direction. An 8 year old might play decent blitz chess, but let him play an online game and he would have no idea what to do with the extra time, so his blitz rating might be lower than his online rating. At the opposite end, as players get older they may not calculate as quickly as they used to, so their online rating will become much higher than their blitz rating.
While your rating gap is 300 points, that does not mean it is typical, and there could be wide variations.

Blitz can also be lower because of disturbances. For example I lost blitz games because the doorbel rang or someone started to talk against me etc.

The only time I ever play live games is with a kid crawling all over me or a wife nagging me. I wait to make my correspondence moves until I can devote the necessary time to study the position undisturbed. I don't take blitz games seriously at all.

In Blitz, I play one out of five openings as White, and one out of three openings as Black.
As White:
1)Grob
2)Hammerschlag
3)1.e3
4)1.h3
5)Bird's Opening
As Black:
1)Fried Fox
2)Philidor's Defense(1.e4 e5 2.Nf3 d6)
3)1.e4 e5 2.Nf3 Qf6
This is probably the reason for my blitz rating(below 1000)

Hi all
I thought maybe I was special but it would seem quite a few people have the same problem as I. My correspondance rating is 1912 but my blitz is only about 1100, I have played about 1000 blitz games but cant seem to climb higher than 1200.
I have noticed that guys at 1800-1900 rating (correspondance) are not actually that strong, they see 2-3 moves deep and often make quite obvious mistakes, however 1800-1900 rating level players in blitz are able to kill me with these same silly little 2 move combinations, I am just not quick enough to spot them with time pressure.
The other thing I've noticed is that the opening theory is usually quite strong with 1800-1900 players, they'll fly throuh the opening where as I need quite a bit of time to look for strong moves and once they've gained a strong position they usually have a lot more time to look for cheap tactical tricks.
I am new to chess, (only being playing for about a year now) and my guess is that players who are stronger at correspondance is due to the fact that our pattern recognition skills are not as refined as the players that have been playing for much longer. Part two is that we do spend alot of time studying chess, playing through grandmaster games, tactical puzzles, openings, end games, positional studies etc and we do learn a great deal but are just so much slower than players who have had more practise.
Compare a calculator to doing math on a peice of papper, the awnser will be the same just took you more time. I do feel though that in order to consider yourself a good player you should be able to play good blitz chess aswell, it is a big part of the game and is where you truly get to test your strength as a player. All the great players calculate quickly and accurately that is the main difference between casual players and pro players. My hats off to the guys who play at +2000 in lighting and bullet games.

Jozi_88, I have a hard time understanding your message as your current correspondence rating appears to be 1034

I have blitz rating around 1500 and online around 1800. The thing is I learned a lot recently about chess which I can't yet apply to blitz games. And playing blitz is a lot about expirience, I do much better in unusal positions when I have time, while in blltz i tend to blunder pieces in time trouble.
Frankdawg: I do not say that blitz rating shoud be equal to online. I am also 300 points higher in online. How can one be 700 points higher? Is not that too much? Can DB give you additional 400 points? I do not think so, but maybe it can.