A few weeks ago, I was having similar problems.  I'd been mining on Slush with good results but it seemed my payouts were starting to get random, then I noticed my mining software (cgminer and bfgminer) was reporting more stale shares than previous and also began random appcrashes.  Yeah, I thought something was wrong with Slush, so I moved to another pool and things seemed to be better for a few days, then it all started happening again there.  Finally, the modem started random dropouts.

Called my DSL supplier and they sent a tech out -- found I was 450 meters further from their local switch than my high speed line rate allowed -- signal was just below borderline minimum and the modem wasn't strong enough to consistently pull it in.  After he called the central office to drop my line rate to the slower tier signal, the modem -- and my mining -- stabilized immediately.  I also immediately came back to Slush's pool.  Now, I'm waiting for more switches in my neighborhood and I'm looking for a better modem, but my only mining problem is how to add hash rate fast enough to keep up with the network.

I'm not saying you have the same problems I had, but I think 21Th/s worth of satisfied miners suggest it's not the pool that's causing whatever is wrong with your results here.  If you check out your end of things and still feel Slush's pool isn't giving you what you expect, then try the other pools.  And if there is a problem with this pool, then Slush is probably working on fixing it -- debugging software and/or adding new features takes time.