Volvo: 86 240dl overheating, volvo owner, volvo owners

Question
I have a 1986 240dl wagon with a 4 cyl. engine (2.3 litre).The car has close to 150,000 miles. The problem I,m having is sporadic overheating. The first time it happened I turned on the heater and the temp. gauge dropped back. The first thing I tried was to flush the system. Next I replaced the thermostat. Then I replaced the waterpump. Then I replaced the thermostat again with an expensive one. It is still overheating and you never know when it is going to happen. I have left for work in the morning when the outside temp. is in the 60's and had it heat up. Then on the way home when the temp is in the 80's it may not bother. The one constant is turning the heater on will drop the temp. Any suggestions? Thanx  

Answer
Well, you listed the two biggest culprits as parts already replacement, and at the moment I can't think of another system that would affect the sporatic overheating like what you've described.  I do have one question, however.  In an overheat, do you actually see the symptoms; like loss of power, boiling radiator water?  The reason I ask, is because it is also a common problem for volvo owners to have a temperature gauge that simply reads incorrectly; usually due to a degrading wire harness under the hood that causes random shorts.  If you don't have a record of overheat symptoms, look into the temp gauge malfunctioning.  If you do have evidence of real overheating, I would suggest you take the question to www.brickboard.com, a free volvo owner's web forum.  Your question will receive attention from the eyes of many experienced owners that might have different suggestions.