Slant Six overheating

I disagree that a stuck closed t'stat will not affect a slant at idle because of coolant through the bypass. Coolant going through the bypass is already hot/heated by the engine, so now the engine gets only heated coolant, & this coolant is not going out to the rad to be cooled. It will overheat.
I think the most likely cause of the problem is a stat that is stuck partially open. Enough fresh coolant flow at idle, not enough at higher engine speeds.