CFab

So I had replaced the thermostat the first time and a couple of months later it was overheating again- usually after it had been running off and on that day. short 10-20 minute trips. doing fine, then temp gauge shoots up and it doesnt come down till I turn it off. The heat might not work as well. Ive burped it by the manual a couple of times since the 2nd thermostat (from Subaru- amazing how much better it is). It is doing it again. It doesn't seem super hot- there is no steam and it smells ok. the radiator may be hot or it may be warm. I may keep the fans running and it'll cool down pretty quickly. burp it again and it does it again. no leaks of coolant or oil as far as I can tell. ANY IDEAS(besides head gasket)?

My 1998 Forester (manual trans) runs great, but began OVERHEATING a few months ago. the heater wouldn't work either. I replaced the thermostat (seriously! that's not the best placement for a part that should be an easy fix). It was working fine for a couple of months and it has suddenly began to overheat again. It will be fine, then suddenly start to climb to the RED zone and beyond. Heater won't put out heat generally when this happens. It rectified itself a couple of times, but then would begin to overheat again. it seems like the coolant is not circulating when this happens as the radiator is not very hot and the upper radiator hose is soft and not very hot either. the engine doesn't feel particularly hot when I hold my hand over it either, and it smells pretty normal. no fluid leaks noticeable. the fans are running fine. It seems it cools down pretty quick when I stop for a bit. What is up? is the water pump going out? was the thermostat i bought defective? please give me constructive suggestions how to troubleshoot this so I don't go replacing parts I don't have to.