I've got a '79 Chevy C-10 pickup with a 305 under the hood, and every time I put a thermostat in it, it overheats. I've tried 3 or 4 thermostats, and I know I'm putting them in the right way, but it'll run for a few minutes, get hot, and stay hot. Eventually it'll overheat and stay there until I shut the engine off, then it cools down and will run cool for a while until it gets hot again. It seems to mostly do it at speeds over 30 mph, and the upper radiator hose collapses when I hit the gas. I've tried flushing the cooling system to no effect, and have tried numerous thermostats. Without a thermostat it runs cold as hell. I'd leave it alone, but without a thermostat it runs like crap and burns a lot more gas. I'm pretty sure its not the headgasket as my oil and coolant look fine, and I've got no smoke coming out of the tailpipe.