Pontiac Repair: cooling system, head gasket problem, intake manifold gasket

Question
Car: 2000 Montana Ext; miles 106K

Great car through 80k miles. Never ran hot, seldom over the first large heat gauge slash mark. A year ago, intake manifold gasket replaced. Since then, when idling or traveling slowly in cold or hot weather, temp gauge climbs
to half mark and beyond. Goes back down once car is moving down the hiway. Never hear the cooling fan kick on, but dealer checked and fans work. Recently had t'stat replaced, system bled. Seemed to help for 2-3 days, now problem back. No signs of head gasket problem, car runs OK, if hot.
Thanks for any advice you may have.    Bruce

Answer
Hi, check the belt and make sure that it is tight, if this is ok, then make sure that the fan is coming on, you can let the engine idle and note the heat gauge, the fan should come on around half way or just little passed, this would be around 200 to 220degs, if the fan don't come on at this temp. then have the fan relay and temp sending unit checked, if the fan do come on, then have water pump checked it could be weak.