Volvo: 240 Volvo Thermostat & Loss of Power, coolant level, volvo owners

Question
Hello Nigel,
I have a 1992 Volvo 240 Sedan with 159,000 miles on it.  I love this car because it has always been so dependable.  In the past year I have experienced a puzzling problem which my mechanic has not been able to solve.  Last summer I was driving in hilly country when the thermostat indicator rose into the red zone. My car lost power and I had to creep up the hills at 10-15 mph with the heater on.  When I stopped at a garage for help the mechanic could find nothing wrong with my car - the temperature seemed to be within a normal range so I continued on.  I took my car to a Volvo mechanic the next week and had the radiator, thermostat, transmission and catlytic convertor checked.  Nothing stood out as a problem.  Debris around the radiator was cleared away, the thermostat was replaced, and a regular servicing was done.  The car drove normally until March when the same symptoms appeared then went away for a while.  Last week I drove in the same hilly country as before and experienced the same problems(thermostat high & loss of power).  When driving around town the thermostat goes up and down and I feel a constant dragging sensation.  I live in a flat valley and my mechanic can't reproduce the problem I have in hills.  He doesn't know what to do next. I would like to keep this car for another year.  Please help!
Thank you,
Vernice

Answer
the only things that can cause problems are: loose belts, water pump seizing and not turning, thermostat, low coolant level, plugged radiator, plugged coolant passages in engine-unlikely since it only has 159k miles. actually this happened to me yesterday, I was driving up a steep hill on the highway at 70 mph, and the temp gauge started getting hot, this should never happen with so much airflow going thru the radiator so i was quite alarmed, so i just slowed down, and the gage went to normal and everything was fine for the rest of the day, the volvo owners manual says that this is possible.