Can anyone help solve this??

Have a recently purchased (2 months ago), 91 Vanagon,
2.1L Auto. AT first drive in the morning, after the
van has sat for AT LEAST 2 days, it stalls after
driving fine for about 5 min. It seems like a fuel
starvation problem. While it is bogging down, if I
press harder on the gas pedal, the idle still drops to
stalling level, while making small backfiring "pops".
It starts right up right after it stalls, but stalls
again after about 5 sec. It does this for about 5
startings/stallings, and then runs flawless the rest
of the day!!

Have tried new fuel filter-helped for 1 day. Tried
injector cleaner-helps somewhat, but it continues. One
important note-no stalling happens when the tank is
FULL, but happens after that, meaning more than 1
gallon from full has been used.

Do I need to drain the tank?? HELP!!!!!

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