are you stacking the clients on top of each other or are they spread over the screen? before i was having the same problem where the gfx card was overheating when trying to run 4 clients on top of each other causeing them to mix artifacts from 1 to the other causing some very groovy patterns! however when i used maximizer to spread the clients around the screen it stopped. that could be that maximizer was lowering the FPs wanted by each client lowering the load on the GPU and therefore lowering heat or it may have just stopped them from stacking. might be worth a go if your currently "stacking" the clients.