Odd temperature activities

Sorry - thought I mentioned that in the initial comment. 225, completely stockI nirmally other than an electric ignition. No mechanical changes at all in the 500 miles leading up to the first occurrence of the described symptoms.



Tracked it with IR all the way up to the hottest I had seen, 205 on the IR.
I tell you man, the dead best thing I did to mine was put a 160 high flow thermostat in it. I doubt you need a 160 but a high flow 180 won't hurt a thing. I normally recommend a Stewart Components thermostat. But the slant 6 head's thermostat port is not quite big enough for one, so here's what I recommend:
https://www.oreillyauto.com/detail/...-car-1968-plymouth-valiant?q=thermostat&pos=1