Welcome game,
Either the archives or someone here will be able to help you. As you may very well know, the cooling systems on these vehicles are somewhat of an achilles tendon. One that I have had my fair share of struggles with.
My guess is air in the system. At engine speeds above XX, you may be pushing an air pocket around, thus affecting cooling system performance (and heating system performance for that matter). Again, a guess. It may be worthwhile changing your coolant and doing a proper bleeding. Have you driven it since the 'overheat' with no unusual behaviour (the car that is) ?
Good Luck,
someone will come to the rescue,
Dave M