Have you changed any drivers in the past few days? Catalyst and SDKs sometimes don't play nice.
I know this isn't necessarily an end-all solution, but if you just really want to start mining with the power you have, try another program. I used to experience similar fluctuations with DiabloMiner. I ended up switching to cgminer, getting better hash rates, more reporting consistency, and haven't looked back since.
Apologies I can't be more helpful.
I don't know, but I don't think so, but I just checked my rate on GUIminer, and its back to about 240-250 mh/s.... Must have been some odd bug or something, because the problem seems to have fixed itself