So I have a 2000 Oldsmobile Intrigue with about 190k miles. It currently has an overheating problem but first I'll discuss some history:
-It had a previous overheating problem a few months ago that was accompanied by bubbling sounds. We diagnosed this as boiling coolant because the system was not pressurizing due to a leak in the reservoir. We fixed the reservoir leak and pressure tested it to find that the leak was repaired. The bubbling noises stopped and the car no longer overheated.
-The car was then underheating nearly all the time. We diagnosed this as a broken thermostat, probably killed by the overheating that was caused by the leak in the reservoir. We replaced the thermostat and bled/burped the system and now the car does not underheat.
Now the car operates around the half-temp mark and stays very regular except in traffic or parked in the driveway. So it overheats when it is idling. You can rev up the engine in these cases and see the temp gauge fall back down, so it is dependent on RPM only and not on the actual traveling speed of the car. Also at this time the car supplies nice heat to the cabin.
Things to exclude:
The electric fans seem to be working fine and they switch on and off supplying a good deal of wind.
The system was bled/burped.
The system has a new thermostat.
The system does not leak based on a pressure test.
The system has enough coolant.
The system supplies good heat to the cabin.
My guess is that there is a clog/general grossness inside the system someplace or that the water pump is starting to fail, what do you think?