I get the same but if you leave it mining for a while, the pool side will stabilize. Keep in mind that pool side values are way off and it doesn't really matter. All you should care is "accepted" shares and you get paid per accepted share and not Kh/s value.
What pool you use?
Yeah, that's right.. Currently I use ipominer.com, where the rate is usually quite accurate and stabilizes after just a couple of minutes. If I let it run @850 for 10 minutes, I see 340 for example, and if I go up to 1150 or 1200, the reported rate stabilizes quickly at a lower level, like 280, where it stays. The accept rate is higher at 1200, but as ipominer is vardiff and bfgminer reports wrong diff "0", I'm not sure if this has any meaning..