I just had a similar problem.  Mine was that the grounding strap on the left
side of the motor on my '85 Vanagon 4 speed has broken.  It would buck alot
and gas milage went south.  Check all your ground straps (one on tranny and
one on right head) for corrosion and good contact.  I replaced the broken
strap with a battery cable from the local Farm and Family for $3.  And the
bucking stopped and the gas milage has gone back to normal.  Of course my van
had just had the cap, rotor, sparkplugs, and sparkplug wires all replaced too.
If you haven't done any of these for a while this could also be your problem.
Look inside your cap at least.  If there is any carbon trails inside that
could be a problem or the cap itself could be cracked.
Have fun.  Good trouble shooting skills are a must when you own one of these
vans.
Thanks
Ken Wilford
John 3:16