If your main bearings were "going bad", then you would know it by audible rod knock. Then you will be stranded on the street for real. Try putting a thermostat in it and see what happens. That's pretty much all I can tell you. It might be something wrong with your headgasket, even though it might not smoke. But you did say it smokes on initial start up? Or your head could have a crack in it, same thing with my brothers car, it had a crack in the water jacket. And lost coolant subliminally.