When the needle began reaching for the red, I changed the thermostat. The cooling problem appeared solved, but just for a day. Soon the needle would go to the red after less than a mile. Since that time, I;'ve used flush and cleaner on the system. Once I even installed the old thermostat, and there was little if any difference. It seems I'm bypassing the real problem. Thanks for your help.
George

Maybe bad radiator hose causing blockage inside? Wash out the radistor fins with a hose. Bad cooling fan clutch? Does it just overheat w/ the AC on? Bad aux fan or temp sensor switch.

Could be cooling system, could be a gauge issue.

Are there any other overheating symptoms? Boiling over? Losing coolant? Dirty white foam in the coolant overflow? Milky slime on the oil filler cap?

Notice any pinging when accelerating, like a timing knock? Hard starting when the engine is hot? Green puddles on the ground?

If none of the above are present, then it is likely a gauge problem, or more precisely, a compensation circuit issue. The temp gauge has circuitry that allows minor fluctuations in temperature without moving the needle. When that compensating circuit goes bad, the gauge will move all over the place.

Here's a quick and dirty test. Next time the needle creeps toward the red, give the dashboard top a sharp whack, right above the gauge. If after whacking, the needle moves back to the safe zone, then it's probably the compensating circuitry. Doesn't always work - but I've had a wandering temp needle in my 89 240 for the nine years I've owned it, and it responds to the whack test every time.