Hello.
Have you tried to use some other miner like poclbm to see if it's miner of connection problem ?
Do you use some kind of proxy/firewall that can break LP connections ?
I used poclbm with GUIMiner frontend before and it yielded about 2% stale/invalid shares recently (but there're no logs to find out what time these rejected shares were at). I switched to phoenix due to a higher hashing rate of 228 vs 216.
I have standard Win7 firewall and also there's a firewall on my 3COM router, but I don't think they would break LP connections. Anyway, I'm going to turn both off and see if anything changes with the stale shares ratio.