Oh I misunderstood I thought it happened all of a sudden.if its perfectly ok when moving but the temp starts to creep up at idle I gotta go back to what has been mentioned earlier not enough either air flow or water circulation(or a combination of both) but you've covered about everything so something that has already been touched on is not doing it's job. If I had to GUESS if I was sure that there was plenty of air flowing thru the rad then I would bite the bullet & get a better radiator(more area,more cores).The more I think about this if it was my deal I would buy a GOOD radiator.