wierd overheating problem

I have a 92 Toyota pick-up. When I turn the thermostat to heat the engine tries to overheat, to correct this I turn it back to the blue (cold). I have plenty off hot air when the heater is on. No one I have talked to can tell me what might be going on . Any suggestions would be greatly appreciated.

Check this site out!! Look under Thermostat!!

http://www.4crawler.com/4x4/CheapTricks/index.shtml#Thermostat

check your heater control valve on the firewall its attached to the heater hose.