The Van:1988 Chevy G20 Conversion Van152,000 miles.
Motor- 305 5.0 V8.

Problem: When l hit the gas pedal my acceleration is up and down and not steady and consistent. Most of the time when accelerating from a stopped or slow position it will hesitate and then pick up again.

l just had a tune up BTW new wires, spark plugs, cap, rotor, egr valve, fuel filter, timing check, oil change etc.. Just recently changed out the oxygen sensor too. l think the issue is the van has been sitting a while and my mechanic basically said it needs to be driven for a while and it should subside, but it is been goin for about 250 miles and still has this issue.

Anyone have any ideas or advice? My only last thoughts on the problem is that it could be improper air mixing or possibly fuel pump but my fuel pump is electric.

could have a vacuum leak, or the gasket under the carb could be leaking and need replacing

could be the fuel that you use since you have replace most of everything it could be already maybe be timing off a bit

The old fuel in the tank if tank was full while sitting well that gas would be kind of spoiled or worn down , yes gas is not so good after time.I'd try adding some Lucas Gas Treatment & run a few tanks of gas through the system then change the fuel filter again, plus you've not mentioned the air filter I'd check it asap too.

Hope that helps & best of luck.By the way it be a good idea to clean the battery , starter , alternator , engine block posts & connections with a small wire brush & check the whole vac system for ant cracks or leaks or loose connections too.

The condition u describe sounds like an intermittent misfire or lean condition, if it powers good on full throttle u can rule out the fuel feedback system. ( o2 & such)
If the condition did not change at all after the ''tune-up'' u can rule out the ignition system. So u r left with fuel pressure, injector problem, or mechanical issue w/motor. You should get it back to the shop or yours, drive it with a fuel pressure guage & compare to spec. / if u do not find it do a compression test & a running vacuum test to rule out valve / camshaft issues.