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