i have a 95 grand prix i have replaced the thermostat, water pump, heater core, and i still get no heat one minute i have a little heat then the next minute i have no heat especially when car is idleing, if i raise the rpm sometimes i can get a little heat excess coolant escapes into the overflow more than normal and keeps setting off my low coolant light?

Sounds like a head gasket has failed. Are you getting a white exhaust.

negative on the white exhaust. exhaust seems normal

After your car has been sitting overnight, go and start it and immediatly squeeze the radiator hose. If it has high pressure on initial startup then the Doc is correct.

BTW, what motor and how many miles? Is it the 3400DOHC? I think these are prone for head gasket failures.

i have the 3.1 motor 3100