So, I'll play for about two hours. I'll win anywhere from 5-20 bb. Great. After a hour and a half to two hours, I'll get major fluctuation. Normally, I'll go all in once or twice and lose more than I made. This is a distinct pattern in my play. I'll play long grinds, win a lot, and lose in the end. My clip and stats get worse the longer I play. Maybe I have a focus problem? Would limiting my grind session time or tables help?
We need to know what the major fluctuations are.
It sounds to me like you just make mistakes perhaps.
Look at a session where this happens, post a hand where you lost your profits. My guess would be you aren't playing optimally anyway. For example, you play sub-optimally, win some $$$, you are happy. Then, it catches up to you, and you get cracked, and lose profits.
Take an example of a session, and post the whole thing. Every hand you VPIP. If you do this, we can really get an idea of how you are playing, and really give you some good answers. We can only guess as of now, because you haven't provided any details.
If you really want to figure it out, this is where I think you should start.