98 sunfire wont warm up -- confused!!!

[1998pontiacsunfire]

Ok I am posting this for my girlfriend.

It started this past summer of 2005 when her car would get poor gas mileage and not warming up to normal temperature, so we replaced the thermostat. The problem disappeared. About a month ago, the problem came back, but this time it was a little different. Now the car will get to normal temperature driving in the city, but after about 50 MPH the temperature goes down below normal and the heater just blows out WARM air, not HOT air. We told her parents it was likely the thermostat again but they didn't believe us. They had it taken to a local shop who checked it over and came to the conclusion that it was the thermostat.

Now, about 2 weeks after taking the car in for service, the problem is still here and we're all stumped. They even took it back to the shop that changed the thermostat and he looked it over again. This time when they got the car back he told them "The engine is just so worn out that its not creating enough friction to warm the car up" which is obviously a bunch of bullshit because it warms itself reving at lower RPM's, why wouldn't it do it at higher RPM's?


So my question is this.... what in the world could be causing the car to not warm up to normal operating temperature?

[unbe]

"The engine is just so worn out that its not creating enough friction to warm the car up"
Wow it would have been nice to have them put that in writing and sign it!