Sluggish engine seems to not be getting gas

I have a 1965 Ford Mustang Coupe, 54K miles on a rebuilt 302 V8 that recently had a good amount of work done on it. Rebuilt power steering. Reinstalled exhaust (had Purple Hornies). Tune up. Among others.

Since I got it back, I have had the same problem a few times. I start up the car and it idles roughly for a bit and then seems to steady out. When I drop it into gear (auto transmission) and put on the gas, it starts and stops and acts really sluggish. I put on the gas at all and it either lurches or outright stalls.

I have had the jets on the carb adjusted twice. It does have a non-stock air filter on it. I did find that the butterfly valve on the top of the carb was sticking and adjusted a pin to allow it to move freely.

I'm at a loss. As my option at this point is to take it back into the shop for more exploratory surgery.

Any help will be appreciated. Thanks.

Dave