I think it's a really good possibility you have excess carbon buildup on the heads and pistons. This causes too high compression and impedes the heads from carrying the heat away.
Too much or too little timing can cause high head temps also, as well as lean jetting as I already mentioned.
I would try the old engine steaming trick. You rev and hold the engine around 3000 rpm or so, and drip water into the carb. You need to pour just enough so you get a huge cloud of white smoke out of the exhaust, but not too much so as to hydro-lock the engine. The 3000 rpm helps this a LOT though. There is little possibility of hydro-lock at 3000 rpm, but you need to be ready with the throttle to keep the revs up.
I have no clue as to why you have sudden high oil pressure unless one of the bypass pistons is hanging up.