I have a 85 300d and the other day the heater stopped working at hwy speeds. It would come back on as soon as I slowed to say 30 mph. I thought it was the thermostat and I replaced the old one. Now it still has no heat t hwy speeds and also now coolant temp gets pretty hot unless I'm in the throttle. Like at a stop light it gets up pretty high and as soon as I hit the throttle the temp instantly drops. What gives?