1) Downclock memory to 300Mhz if you haven't already
2) If you're not using extenders, then you need to space out your cards a little, jam some foam peanuts, bits or plastic or something around 1/4" thick between the cards if they're too close.
3) Set fan speed to at least 60~70%, or 100% if you can tolerate the noise.
agreed, did the same thing to reduce my 5830's temp.
also replace the TIM.