Volvo Repair: engine overheating, temp gauge, operation temp

Question
I have a 97 volvo 850 turbo. The temp gauge goes from normal to hot back to normal. sometimes if I set the air temperature above 75 the temp gauge will return to normal. If I run the air conditioner, my water will steam out from the holding tank. I have good coolant in the radiator. It seems to me either the thermostat is bad or possilby my water pump. What do you think? thanks

Answer
Hello David,  Well the first is the t/stat,if its not been replaced that You know of,it time, also replace the temp.sensor that is mounted in the t/stat housing.Its been stressed and it will set off the CEL.As for the water pump,that should be relaced at the next time belt interval.Along with that the 'idler pullys and t/belt tensioner,all due to age/miles.We have seen that by the second interval they have started to fail and it toooooo costly to 'bet on'..Eight years and possibly 140/150 mls.is 'safe life'.Don't overlook the aux.fan for proper operation:Temp.and a/c activate.Even if the h2o pump is dry,Replace It!