I hate to say it since i've been playing for a while but i'm still a complete amateur, and it gets really tempting to get caught up studying openings like I did yesterday (Chigorin, General Sicillian, Najdorf, Queens Gambit Openings, Indian Systems), on a side note however the theory I did studying sicillian (even if it was the najdorf theory) actually helped me beat Shredder on my phone at 1600 elo today in a Reverse Sicillian/English since I remembered some of the theory for whites side in the sicillian.
Still I have to agree with Pfren, when it comes to tactics and to a lesser degree general chess theory, i'm really sloppy, and it's probably what hinders my game the most.
*edit*: I forgot to mention that I myself tend to notice this when i've studied, the massive drop off. It's just rare for me in this case to actually remember and apply it to a game like this a day later.
I've been doing the same thing: studying hard and then finding myself losing to lower rated opponents. It's pretty discouraging.