Water pump cavitation is a definite possibility. Patricks has information on this, must be in his print catalog. Go to the links section to his website and e-mail him. What this involves is using a largeer pulley on the pump, slowing it down.
Another issue on water pump cavitation that I have learned from the V8 world. GM pumps have too much gap between the impeller and the block. A solution for this is to rivet a plate to the impeller, narrowing the gap, not allowing the water to get sucked underneath the impeller, resulting in cavitation.
Another possibility is valve adjustment. I went with the M4F cam from Patrick's, had it adjusted per the cam card. Patrick recommended different numbers, which lowered my temp at highway speed from 190 to 180*F, but you are far beyond those numbers.
