I have an 88 Grad that is over heating. No oil in coolant or coolant in oil. At first the temp guage showed it was getting hot, then suddenly it dropped back to normal. I replaced the thermostat. Now it seems to only overheat in stop and go traffic, and runs normal at freeway speeds. The car has A/C, but no drive belt. The fan belt is good and so is the tension. Any ideas?