Don't run your car without a thermostat. The cooler the engine runs the faster it will wear. I'm guessing your just running parts store thermostats if your having this many go bad, or you have some other issues that need to be addressed. Maybe some of the other guys will chime in with some suggestions as to what your problem may be. Either way, get yourself a good thermostat like the 180 degree Mr. Gasket ones. They seem to work well and are highly recomended around here.