Q: Overheating

Started happening a few months ago. The van suddenly overheated. We changed the thermostat thinking it was the problem. Still overheated. The head gasket has been replaced on this van already, but it was a while ago. Not positive of the total miles on the van right now, but I'm guessing it's pushing 180,000. The gasket was probably done at least 50,000 ago. Took it to the garage multiple times, they can't figure out whats going on. Doesn't seem to be loosing coolant. They've bled it multiple times, and it was ok again for about 1.5 - 2 weeks at a time a couple times, then we had them flush the system on advice from other people, and it seemed to do the trick and was not overheating for a couple of months, but the heater was not working great, mostly blowing lukewarm air. We decided we could live with it as long as the van was no longer over heating. Unfortunately, it overheated again tonight! Will be taking it to the garage again tomorrow, but not too optimistic. Advice?
My car has an automatic transmission.