Runs Rough at Idle?

[68longhorn]

My truck is a 1986 c10 with a stock 305 and 700r4. Recently the truck started idling rough, sometimes feeling like it wanted to stall. As soon as you raise the rpm a few hundred it runs smooth again. The spark plugs had 50k miles so I changed them but still the same problem. I sprayed around the intake with carb cleaner but there are no indications of a vacuum leak. I performed an idle vacuum test, truck hast 17inches of vacuum and the needle is steady. I checked timing and it is at 8 degrees at idle with the vacuum advance disconnected. In the last 5k miles I had a distributor fail and a fuel pump so those are new. I tried upping the idle speed screw a half turn then the truck wanted to diesel on hot shut offs. What could cause it to one day start idling rough with no other concerns?

Also today I drove the truck 225 miles to a show running 75mph and when I exited the highway the truck would not idle at all. I spent a few hours at the show and when I left it idled rough but ran fine, and after the 225 miles home it would idle rough but not stall like in the morning.