typical partly plug rad will show up at higher RPM's (eng producing more heat/rad cant keep up). I'm trying to decipher what is changing but nothing is jumping out at me. You're sure nothing has been changed/altered? It ain't making sense (but we love the toughies!). If it was OK WO a shroud then it oughta stay that way but SOMETHING is slowly changing/deteriorating & that is what we need to figure out. The cooling system will hold psi when hot/set overnight?
Partly plugged rad can show up at idle also, my 67 Charger always got hot in traffic, parades, in and out of heavy car show parking, had rad re-cored last spring, last year coming out of Moparfest, in line for almost 1 hour, never got over 185 on a 90 degree day.
