[cadillac_al]

I don't think any of my trucks ever had a fan clutch. I figured the General didn't put them on trucks. (?) Your problem sounds like the issue I have been having with my motorcycle for many years (I know, completely different). It cools fine as long as air is moving through the radiator but in slow city driving it will overheat in a minute. I finally ruled out everything but the radiator. I think that is the trait of clogged radiator; overheating quickly as soon as the air flow slows down. I would bet my money on a bad radiator. Good luck.