This may be happening because your Internet connection isn't fast enough to handle the high rate of share submissions. Two ways to improve this situation:

• Make sure you use the Stratum protocol to connect to your pool over the Internet, and not the older, less-efficient "getwork" protocol. If your mining device does not support Stratum then run a Stratum proxy (bfgminer or mining_proxy) on your PC and point your mining device at it.
• The pool you are mining in may have a setting to adjust the difficulty on your miner, or it may auto-adjust it (a.k.a. "vardiff"). Try increasing the difficulty for your mining device the rate of share submissions is decreased to a more manageable level.