Here's a thread from the Syncro list about this problem.   Someone also
posted recently that an inexpensive Radio Shack capacitor wired into the
air flow meter circuit would fix the problem..sounded like a good fix...but
I couldn't find the post with the instructions... will keep looking.

The  thread:

> Howdy,
>
> Saturday morning while driving on the freeway I
> noticed a sudden bucking in the 87 syncro. The buck
> happened a couple of more times while on the freeway,
> then ceased until I got to my destination. While
> comming home that night, it started again (at freeway
> speeds) and happened three times within about 5 miles.
>
> This morning on the way to work.. bam- there it was
> again at freeway speeds (65-70). I slowed down to 60
> for the next 30 miles and it didnt buck again.
>
> The buck is kind of hard to explain. At first I
> thought it was more like a power surge - but how much
> power surge can I be getting at 70mph? It almost
> feels like Im hitting a bump... The rear of the van
> seems to almost lift a bit, I think I can hear a clunk
> like sound that seems to be from the rear. I havent
> down anything back there since doing the CV's about
> 5000 miles ago. Only other things Ive done recently
> is rewire the tachometer and replace the battery -
> both the night before the symptoms started. I know my
> throttle switch is bad - one of the screws is broken
> and I idle high and sometimes have a buck in first
> when I let up on the gas - but this new buck is always
> at 65+ with the pedal not to the floor, nor am I
> letting up on the pedal when it happens. Its pretty
> significant and a bit frightening..
>
> Any clues?
>
> Thanks
>
> jeff
>

I have experienced this symptom and it has been discussed quite a bit
in various Vanagon groups. It usually happens in dry climates at
freeway speed. (Mine occurred while driving in Arizona) I seem to
remember it was caused by static elect building up in the air flow
meter and shorting out the wiring communication between the ECU and
the AFM. VW was also aware of it and sold a factory part to fix it.
Any good VW shop or dealer should know about it....or search
vanagon.com for "bucking". You'll get lots of hits.

The VW factory part is a wiring "pigtail" that connects (if I
remember correctly) between your air flow meter and the normal
wiring to it (computer wiring??). The part is fairly expensive from
the dealer (surprise!....est $125) but I've also heard that a simple
solution is sometimes achieved with a good ground wire to the AFM to
discharge the static.

Warren C.