Is the anti-freeze rusty, could engine need flushed? If it isn't coming out of radiator are you having to add any periodically? Could mean it's being puuled into cylinder during inake stroke. One way to check for a bad head gasket is to fill radiator up to neck, start motor and watch for bubbles getting increasingly larger and more frequently. Engine should be cold of course. Is there any reason your not running a stock housing? I can't lay off the thermostat need because it caused me to go through everything before I checked the most common. Even after a greybeard told me to change it.