$5 says it is the PSU. I had a similiar issue using a OCZ Gamestream 700 watt PSU, great PSU, but I was having weird instability issues. Swapped out for a Gigabyte Odin 800 watt PSU and the instability issues went away. I swapped out my 2 HD4850's for an HD4870x2 and started having problems with the new card. Connected it to it's own PSU and sure enough the issues went away. This was of course after ruling out other possible problems, such as drivers, etc. I have a lot of components connected and I am pretty sure I am pushing the limit of these PSU's. I will probably eventually upgrade to a 1KW PSU, but that's for later.