I do have that problem. One thing to remember is that you will play differntly on different days. You may be more awake, you may be ready to sleep, guess which is more condusive to chess? you may even have a hangover or worse yet be drunk.
Anyway those are usually small fluctuations (though people do have "on" days where they can beat just about anyone). One thing which I find helps is to use the notebook for each game here. It's usually short stuff like "Q is pinned", "control d5" or "chase the knight away before recapturing B". The most important ones are the thematic ideas around which my game revolves, controlling a diagonal, increasing teh scope of your minor pieces or a concerted attack against f7. This helps me resume my anlysis of the game from where I was instead of starting anew every 3 days.
However, to address your question, I tend to do my studying in phases as my motivation comes and goes. When I start to notice a new idea I may often focus on it to the exclusion of my past knowledge and my rating goes down. As I become more accustomed to the ideas I don't focus on it to the exclusion of all else.
I was recently studying games where one side had a color complex. In every game for the next month, I would look for similar ideas and lose to simple tactics. however, in my most recent OTB tournament, I looked at the board on move 12 and realized that the dark squares around his king were weak. So I formed a simple plan
1. Exchange off his dark squared bishop so he cannot defend the weak squares.
2. infiltrate with Q, R and Ns and mate.
I then took time to investigate what my opponent could do. He had no immediate threats so I used the next three moves to force an exchange of his dark square bishop for a knight. He was starting to generate queenside counterplay so I took another look at the position, made a couple of defensive moves which included getting my other knight to d5 and my bishop to e4 (where it defended my king and controlled the a1-h8 diagonal). Then I moved in with the queen, things got interesting and I offered to sac a rook. After much manuvering, he managed to defend, but had to exchange queens and allow my rooks onto the seventh rank which eventually won me the game.
The keys for me were:
1. Noticing an appropriate time to apply my new idea and not forcing it into every situation.
2. Taking time out of my operations to shut down my opponents counterplay!
3. Making moves which were both offensive and defensive at the same time!
4. Being willing to exchange my current advantage (dark square complex) for another (rooks on seventh).
Wow thats an essay isn't it.
I usually get worse before I get better, but when I rebound, I am better than before.