Can't keep a stack

[college kid]

For a while now, I have had a weird thing happening to me. I play on Party NL $25 buy in tables and my dorm game and dorm tourneys. Whether in the tourneys or cash games I have trouble keeping my chips once I double my buy in. It seems like I always dwindle back down and always cash out just a little up or get blinded out of the tourney. I don't know what is causing this and it's not something that I am used to happening.

I have really thought and though but I don't know why. I don't think I get stupid agressive and I think I play just as well as when I was previously building up my stack. Sometimes it's not just double my buy in--but there is always a key point where I just start to dwindle back down. I am fairly sure that it's not just variance--it happens like clockwork. I am only marginally ahead for my tournaments and live games during these periods.

Has this happened to anybody? Is this the right forum for this? Would I get other helpful responses if this was on another forum?