Do you have an instant read thermometer that will goe to 200 degrees F? If so, use it to measure the temp of the pipe near the coolant temp sensors when the car is warm. Sure, reading 'through' the pipe may not be the best thing, but these engines are hard to get access to the hot coolant without making a mess of everything. So do the best you can with what you have.
I figure if your engine is really running cool, without flogging it, then the independent thermometer should show the area around the temp sensor as much cooler than the 185-195 your thermostat should be keeping. If you see something in the neighborhood of the upper 170's, then I figure you have either a guage problem or possibly a poor grounding situation between the engine and chassis causing the gauge to read low.
If the thermometer reads lower than you would expect, then a partially open (read bad) thermostat could be the culprit. The symptoms you describe sound like a poorly performing thermostat, i.e. too cool during low demand, and too hot when the heat is on.
Brian