Engine Mechanical problem
1987 Pontiac Trans Am V8 Two Wheel Drive Automatic

What would happen if the timing is off one notch? Will it overheat?

The ignition timing is controlled by the ECM and is considered no-adjustable. If the timing hasn't be tampered with, I would say you have another problem elsewhere.

The car is running hot after driving 65 mph or faster for 5 minutes. Under 65 mph it runs fine. The car has had a new timing chain installed and I was wondering if the chain was one tooth off (misaligned) could that cause overheating at high speed? Or maybe an obstruction in the radiator? The water pump was changed as it started leaking (the water pump was bad). It still over heats. What could be the problem?

Cam timing being off a notch would cause more performance problems than anything else. Did the overheating start after the new timing chain was installed? At this point, I would think your radiator is plugged.