Ok, so I have a 91 F-250 302 that's got a rather strange issue. While drivng, the truck runs great, no issues, happy to go. However, if I rev the truck up at idle, the going up is fine, but the coming down is rough. That is to say, the truck revs just fine, perfectly smooth, but stumbles when returning to it's idle. Once it smooths out, you could damn near balance a quarter on the intake, it's nice and smooth, it's just when coming down. The truck also stubles when going from a drive gear in to neutral/ park, but just for a split second. The TPS, IAC valve, FPR, and throttle body are all new within the last year, and the truck doesn't have any codes. My initial thought would be the IAC, but I've tried two, both from different manufacturers, with no change. Something else that I've noticed, is when I put the truck on a lift at school to work on it, it takes much longer to start up after coming down, almost like it's lost fuel pressure. The vacuum line coming off of the pressure regulator is not full of fuel like they often fail, but it does smell like gas. This is my first one of these old girls, so I don't know if that's normal or not. I'm going to do a pressure test next time I'm in the shop, but until then, have any of you seen something like this before? What did it end up being?

Thanks for your time