Now is the time of year where we start seeing cooling problems
Is the coolant level decreasing?
Are there any signs of coolant on the ground under the car when has been parked?
Check the water pump area carefully for signs of leaks.
Is the ignition correctly advanced? Is the vacuum advance working?
Water pumps usually start leaking around the drive shaft when the seal and bearing becomes worn. A leak will mean that pressure cannot be maintained in the engine causing local boiling and significantly lowered cooling effect.
If the coolant level increases rapidly in the header tank when the overheating starts, then the water is boiling in the engine and there's a pressure problem. If you run without the engine hood, it is possible to keep a constant view of the coolant level in the tank in the rear view mirror!
I don't think it would be the drive belt as if that was untensioned it would squeak and the charge light would be blinking.
- Anders