Thanks for the replies. Nope, the water pump was not changed when the timing belt was done. I'll try what you suggested bugman1964. So far I have not seen any oil in the coolant or coolant in the oil. I'm going to be changing the oil soon so I'll know for sure in a couple of days.
As I was thinking about it more last night it seems to me to be an internal leak in the headgaskets. Causing exhaust gasses to get into the coolant which would cause air bubbles and after an 1 & 1/2 hours of driving the air bubbles build up and impede the flow of the coolant. And stopping for 5 minutes allows enough air bubbles to escape to allow the cooling system to function again.
I don't know, does that seem to make sense to anybody?