---

It's difficult for me to imagine running an engine at that speed for that long. I guess if one is commuting a long distance it would not be unusual.

If the cooling system is not in perfect condition I would expect the engine to be running at a temperature above optimum after operating at such a high RPM continuously for an hour. I read that the impeller was replaced but was the wear plate also replaced?

If it was August I would suspect that warmer cooling water temperatures could be a contributing factor but I believe water temps there are around the low to mid-seventies.

Have you tried pumping the squeeze bulb in the fuel line when the RPM drops? If that works you will know you have some kind of fuel flow problem.