Is the return hose from the motor to the reservoir hot? If not there may be something (rust, dirt etc.) Clogging it up. Also could be a bad head gasket, though there may not be signs of a bad gasket.
Just had the same problem with my '95 Grand Am a few month ago. I changed out the thermostat, heater core, unplugged the return hose from the motor to the coolant reservoir, and checked to make sure the cooling fan was working properly. All in all it ended up being the head gasket.
Saturday, February 18th, 2006 AT 4:36 PM