First thing to try, is burping the cooling system.. air pocklets seem to like to be trapped in the heater core. To burp it, just raise the front of the car on stands, then remove the rad cap and let it idle until it comes to temp...
After that I would say its the Vaccum Switching Valve on the firewall of the engine bay. If that went bad, you could try tio find a replacement, or do like I did on my Blue car, and make a manual one (on in the winter, off in the summer)
After that I would say its the Vaccum Switching Valve on the firewall of the engine bay. If that went bad, you could try tio find a replacement, or do like I did on my Blue car, and make a manual one (on in the winter, off in the summer)