my 60 started overheating this past weekend (not all the way, about 3/4) while on the highway or sitting at idle. it would cool down when i turned on the heat though. so i flushed the system today and now it just over heats period. highway, town, idle, etc. turning on the doesn't work now either. i suspected it's the radiator, but now it just stays pegged (not in the red, but it's much closer than before the flush). is it the thermostat?