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?