I have a problem with my VOLVO 960 wagon during mountain driving only. Climbing long steep mountain highways for more than 2 miles at temperatures higher than 90 Fahrenheit leads to a sudden and rather fast increase of the temperature gauge reading into the red region. The only way to get the temperature reading back is to switch the AC off and to start the heater, which is rather unpleasant at those high summer temperatures. I have as well the impression, that under those conditions the car looses quite some power and becomes more noisy. My car is 4 years old and did not show this problem when it was new. The dealer was never able to reproduce the problem. Indeed, driving in heavy traffic in the summer with a lot of stop and go and idling in front of traffic lights while having the air condition on leaves the temperature gauge very steady at its normal condition. This is even true with temperatures above 115 Fahrenheit. The dealer has replaced the thermostat and the cap of the coolant reservoir, but without any change in this behaviour. The problem was checked several times, but without any success. No readings were ever found on the car computer.

Can you give me any advice on what this problem can come from. This behaviour is especially strange, because I have never seen the temperature gauge of a car moving so fast from normal to red (about 30 seconds). In case of such an event, I have always seen the fan running, and the dealer has checked that the second speed works, and the hoses were hot, but not rock hard. I have never seen coolant coming out anywhere.