i didn't mean to hijack the thread but the problem is similar.
brivers: i've replaced the tstat and like rjm1000 i get intermittent symptoms. i noticed today that the rad is cool even when the guage is showing hot and the fan is running. this indicates to me a blockage but it only happens sometimes as far as i can tell. running down the freeway on the flat, the engine heat is fine though generally a bit cooler now since putting in the new parts. the heater only makes warm air and it used to be a furnace.