Help With Hestitation/Stumble Problem - Thanks

Been struggling with a bad hesitation/stumble problem. Had posted previosly on this.

After changing intakes gaskets (may just be a coincidence) starting having a bad hestition/stumble with car when WOT or giving some extra gas when accelerating.

Took to local Ford guru - he called me told me he got rid of the stumble. Went and picked the car up and not a mile down the road starting having the same problem. Took it back - he swore it was running fine for him. We played with timing and springs on the the vacuum advance took for short test drives - about a block seem to run good not a speed demon but pretty good. Now one the way home same thing - 1/2 mile out same thing - hesitation & stumble with backfiring thru carb is back. Whatever the problem its seems okay on these short spurts as we were tweaking but problem comes back when it's driven more than block test drives we have been doing. I'll check timing tomorrow to seem if it's still where we set it tonght but really could use some ideas on what may be causing it.

Some background on the car:
1. New Holley, fuel lines, sending unit, choke is fine
2. Pertronix ignition
3. Good vacuum 20 psi
4. Spark is good tested that
5. Just put new vacuum advance thought that may be the problem

302 motor, 1970 stang.

Appreciate any help - starting to run out of ideas.

Thanks

That is where we started. But if it was timing seems like problem would be obvious off as soon as you drive it (1st block). Like I said 1st block - test drives were okay - problem surfaces after you drive it a little longer distance.

Thanks - I'll look into it. Would that cause the problem to get worst as the car runs more?

I did check once - could not find the little plastic feelerthat cam with it - but used a metal feeler gauge and gap was correct .030 - I'll check again to make sure. thanks

tank is pretty new. Replaced lines, sending unit, fuel pump and put a new paper type filter in. I think it's the distibutor and not advancing correctly. When taking for test rides - short distance - was running best at timing at 2 B. After a mile or so starting backfiring thru carb due to retarded timing. I Think PaBear is right - but paln on repacing dizzy instead of fixing old.