I wouldn't think it would be the thermostat either. When our vx needed a thermostat it was consistantly running about 25% cooler than normal operating temp. Thus the Stat was opening too early (too low temp). Aweful to change stat by itself.

Your problem sounds like it would be the water pump, because of the changing with the rpm's and loads. What is the condition of your Serpintine Belt that drives the pump?

Good luck