could be as simple as a bad cap not holding psi in coolent system the suction valve falls in to the rad and wont hold psi at all.bad water pump or t stat also. a head gasket would smoke out your ex and probly have coolent leaking out ex. rev you engine and watch the coolent hoses if any colaps that could be it.you could try super cool bg adative.or flushing the coolent system.any leaks?