I have a 2003 PT Cruiser GT that had the manifold gasket replaced at my local mechanic, after getting my car back it is overheating when I get on the highway but not around town. I have replaced the thermostat, flushed and replaced the radiator cap but still overheating (fan is constantly running). Could the manifold gasket have been installed incorrectly as I have read that the gasket must be installed a particular way as there is a soft and hard side. Also had a carbon test done to determine if there was a head gasket problem and it passed. What else might be causing the problem.