Cadillac Repair: over heat, head gaskets, radiator cap

Question
i have a 1998 sts it likes to over heat when I'm in idle but when i step on gas temp gage goes down.driving it is ok just as long as it is moving.when in idle both fans turns on but doesn't help.replaced thermostat and water pump tried to open coolant reservoir and rev gas see if there is air, doesn't work and i see no leaks.
is there anything eles i can do please help..thanks

Answer
sounds like a small leak in the head gasket or gaskets. Pull all the spark plugs and apply 80 lbs of constant ait pressure into each spark plug hole and see if the coolant blows or bubbles out of the radiator cap opening. If it does, Replace head gaskets.