1994 Volkswagen Jetta 1994 Jetta misfire/stumble on light a

I have read many different forums and still I have not found a solution. Maybe you can help me?

1994 jetta 2.0 5-speed GL with a/c. Bought the car from my father-out-law, it has a known hesitation/misfire/stumble. It is at 160,000km (100,000 miles) and is generaly very well maint

hesitation / stumble / misfire happens only above 1400 RPM (both with driving or in neutral) but goes away if you open up the throttle to more then about 60%. Only tends to do it while light acceleration, or cruising along (at all speeds) does not do it while hard acceleration. Say you have your foot in a fixed position that allows you to travel at 50 mph and you keep your foot there, it will stop doing it as the road goes from flat into a hill.

one more thing. Say you are cruising at 30 mph, and it is doing its hesitation thing, if you turn the key off and right back on (kinda like bomb-starting) problem goes away completly for about an hour or until you park and turn off.

my father-out-law thinks the problem seemed to start shortly after getting a new fuel tank in the car.

since problem has started we have ;
-changes plugs (2 times), wires (2 times), pcv, O2 senor, checked fuel pressure, replace MAF, even tried swapping the computer