Classic K20 Running like crap. Any advice?

R2424243

Truck has a 502BBC, ran great during acceleration, at stops or slowing down it would die & real sluggish taking off from the stop once i got it running. Seemed to run good idling but once I put it in gear, got pukey again. Changed all tune up stuff & fuel pump. Ran good for a few min then right back to the sos. It has an OLD analog projection 4 fuel injection system on it. Kept getting worse & worse. Took it to a shop, they couldn't find it, they ck vacuum leaks, timing, etc. Said they would get it running great & a few min later, ran like a dog. I picked it up. Now when I slow down or stop it stalls but now makes a popping noise when it dies while I'm trying to give it gas. Doesn't want to start again. Not a cranking noise when trying to start, just a "wrr, wrr" but if I keep feathering gas pedal, I can eventually get it started. if I can get her going, it runs horrible but it will run once I get it up to about 15mph it will usually stay running but no power. Can't get it up to speed b/c it's running so bad. Wont run in idle anymore. Just runs horrible, dies constantly if i stop or slow. Seems like something is restricted or something. No cat converter. no power. never been in such a pickle. Any educated advice?