A failure in that alternator red wire is a common cause of this type of
problem. It can even cause the cycling of the fuel pump symptom. Other
related wiring failures can do it too. Here is a link to a way to figure
out if that is the problem without spending a lot of money and time
replacing major parts that don't help.

http://www.thesamba.com/vw/forum/viewtopic.php?t=349429&highlight=

Mark

Richard Koller wrote:
> I had these symptoms in my '91 and it turned out to be the big red wire on
> the back of the alternator ( coming from the battery I believe ) this wire
> was partially broken. It was not completely broken and you could not see the
> break while it was connected. So it made contact most of the time and the
> van would run. This event happened suddenly on a long road trip and It got
> worse and worse during the ride home. Finally the van just stopped about 20
> miles from home. I had it towed.  I started examining all my connections and
> when I unbolted this big wire from the alternator I could see it was
> severely frayed. By the way the tach would abruptly drop to zero every time
> the van conked out.
>
> Hope this helps.
>
> Rick Koller
>