GMC Repair: repair, core radiator, fan clutch

Question
I have a 2000 somoma 3.1. First thing of the morning as the truck heats up everything is find but within a few minutes the heat turns cold and the temputure gage goes almost to the red.but as I drive everthing goes back to normal.heat back on and the temputure goes down to normal.I`ve changed the themostat twice this year.But it didn`t help Any ideas. Thanks

Answer
Hi
I would check for low coolant, coolant restrictions (hoses, heater core, radiator, etc..) they can plug up. Squeeze hoses, they should be able to squeeze with hand pressure (not hard or brittle feeling and they should also not be like putty). Restricted coolant flow may cause problems as you describe. Also check the fan clutch operation. Check water pump flow. I would also recommend, if nothing found, take it and have the cooling system pressure tested for and leaks, etc...
Hope it helps