That is why we all suggest thrwing an impeller at it. If this is new to you it
should be done anyway, just so you know where you are at but it might be your
whole problem. I have an example in my garage of a bad impeller that looks
perfect. The hub spins and that was RPM sensitive. It drove me crazy until I
just made up my mind to replace it.
A thermostat is also not a bad idea. They are cheap.
One thing you could try is have someone feel the discharge water when it says
it is hot. If the water is "MacDonalds coffee hot" you are overloading the
cooling system. (not enough water pumped, lean mix etc) If the water is cool
you probably have a sluggish thermostat. That is not likely if the engine runs
out good and then fails when you slow down. If it is a morning sickness problem
the thermostat is suspect.