This is what I was going to ask. There's bound to be a block that increases your variance. As I've been saying, average payout is a function of time, and block resolution is all about luck. If you're still ahead of the pool payouts though, this should be a particularly concerning issue.
I've been charting my payouts, and as I've said, the difference between pool mining and solo mining is light and day:
http://i.imgur.com/II9yPst.pngI switched to solo mining on the 13th. The longest gap between block found that I've dealt with was 8.5 hours on the 13th, and I still came out way ahead. It might be helpful to write down some numbers to see how ahead you are.
