Doesn't sound like you're doing anything specifically wrong...I agree that starting hand selection may need some work (but then again I haven't yet migrated over to party).

I've had issues with losing big pots after I've built up a nice stack as well. The solution I came to was to cash out once my stack reached a certain point, just for my own comfort level. Just because you're buying in with an amount that's proportionate to your BR doesn't mean that the amount you have on the line after winning a lot is still proportional.

- Jeffrey