I have three 7950's that will run 24/7 on Bitminter using GUIMiner.

When I setup GUIMiner to connect to BTCGuild, the three cards will work for 10 - 30 mins and then the hash rate drops to like 1.5Mhash/s - 600 khask/s.  It will continue to mine that slow.  Any ideas the reason for the crash?  I'm not using any extra flags on bitminter or when I try BTCGuild.  I have tried some flags when connecting to BTCGuild because it kept crashing, but didn't stop it from crashing. 

I am also on 12.8 AMD drivers.  It crashed using the latest and 13.1.

I want to be able to mine on BTC Guild.  Suggestions?

I read there is a paperclip trick to get a power supply to turn on without a motherboard.  I want to connect a power supply to an additional three 7950's connected to two different motherboards which already have power and video cards.  Ran out of connectors from the current two power supplies.  So, if I get a single power supply which will only be connected to three 7950's, how many watts should that power supply be?  

I was thinking about a 750 watt? or maybe 800?