I have a 2000 Deville DHS with only 80,000 miles and it is overheating but only when driving in the city for more than 20 minutes. It has not overheated on the highway and it does not overheat in park, the tempature will actually go down when I put it in park. I have changed the water pump (3 times), Themostat (twice), presure checked the cooling system and it looks like the fans are running. My Mechanic suggested having the radiator flow checked but I don't think this is the problem. If coolant wasn't flowing wouldn't it overheat in park? He has looked at the system very closely and he is fairly certain thet it is not the head gasket, which I'm sure everyone will jump to the conclusion with. Actually it is difficult for me to get an objective opinion because everyone jumps to this concluion. Today I turned on the air conditioning with the fan on high and although it ran hot it did not go into the red zone like it usually does. HELP!

 

I tend to think head gasket too. You get air pockets and air-pocket don't cool.
Have you looked at the oil? I saw it right away on my car. It looked like a milk shake

 

Just changed the oil last week and it looked fine. I also replaced the fan relays this weekend but it didn't fix the problem.

 

I'd guess failed head gasket/pulled head bolt threads, have a shop with a gas analyzer measure the amount of hydrocarbons in the cooling system after it acts up. This is how I diagnosed it on my 1996 Deville and 2000 Seville. The HC reading in the coolant tank spiked to about 400 ppm in 3 seconds, a passing tailpipe reading is typically less than 8 ppm.

 

Just changed the oil last week and it looked fine.

 

After my 1 st overheat I checked and changer the oil. Perfect
Though I was fine
My second overheat was 30 days later. Then the milkshake oil