1993 Mercury Cougar wont kick on

So is the car getting too hot? Or do you mean when it is up to correct temp? The heater issue could be a few different things, you may have partially plugged heater core that isnt allowing the coolant to flow through it under low rpms or you may have a blown head gasket that is introducing air(exhaust) into the cooling system creating air pockets that go away when pushed around by higher rpms, have seen both do this. If the car doesnt overheat, you may want to have the heater core power flushed to be sure coolant can flow through it easily. Be careful when doing this, those heater cores had some leaking issues and are very difficult to replace(requires removal of entire dash). If car overheats, have the cooling system chemical tested for exhaust gasses.