I'm having some strange overheating problems with a recently acquired 1991 Prelude, 173000 miles. the car will perform normally in city driving and short trips, but when I drive on the freeway for about fifteen minutes going about 80 the engine temp will begin to rise. The strange thing is, that the only thing that seems to affect it is if i put the car in neutral and coast for about 20 seconds, and then either rev the engine or put it back in gear and the temperature will go back down! also the amount that it goes down seems to be tied to the length of time that I coast; longer coast=more temp reduction. This will work for maybe another 7-10 minutes before the temperature rises again, and I have to repeat the process to make it to my destination. using the heater on full blast will prevent this from happening as Fast, and makes the coasting/cooling process faster, but doesnt remedy the problem. Any ideas?

I've had the car checked at a shop, and they can't seem to find anything wrong. I recently changed the thermostat, and I also flushed the system and made sure about getting 50/50 coolant in there, still no change

--also recently my engine decided not to startup, no turning or anything, and I tried to jump it but it doesnt seem to be tied to the battery. The car had been running fine prior to this, and when it happened I was trying to start the car and the engine turned slowly a few times before giving me nothing at all. Any help with this as well?