My 1984 Pontiac Fiero overheated this summer and wouldn’t restart. The diagnosis at the time was that it needed a head gasket. After towing it home, it sat in the driveway until the battery went dead. Then in an attempt to either fix or sell it, I had it towed to a mechanic who got it running again. The car doesn’t overheat … very often. So, it would seem that the problem is not the head gasket but something else. The thermostat has been replaced already so that is probably not the problem. I assume that it is either the water pump or a plugged radiator.
Apparently, other people have had similar problems with the 2.5L Iron Duke four-cylinder engine. The first Google listing for overheating Fiero sent me to an online tractor forum. Go figure that one out. The suggestions were anywhere from ignorant to helpful with many others in between. I include the link below for your perusal.