My Guess is an air leak into the fuel system.
What filter are you using? Large rectangle on the firewall?
My 90 6.2 started like that after i got it on the road.Turned out one of the plugs on the filter base was almost corroded through.Wouldn't leak fuel but would suck in air when it wasn't running.It was the rectangle one on the firewall.
I ended up pressurizing the system and found the leak then.
90 Chev 3500 c/c 4x4,6.2na,400 auto,4:10 gears.DSG Timing gears,main girdle, isspro tach, pyro,boost,oil and trany temp.Dual Tstats, High volume peninsular pump,on shelf, Custom turbo and intercooler 85%complete. Change of plans for the dually, it's going to get a Cummins. Both trucks are Blue 90 4x4 crews