The car seems to run fine above 2500rpm, but getting the car above that speed is a problem. Most of the time, we're traveling at speeds which equate to engine rpm of 2000 or less. There seems to be 3 coolant sensors/senders on the engine where the upper radiator hose (and I suspect that this is where the thermostat resides) goes to the engine. Is there a resistance measurement or other method of determining a bad sensor or should I try to find the parts online? Thanks for your diagnosis, it's a little more comprehensive than I had anticipated.