Sounds like a bad ground for terminal 13 on your computer.

Ground for this wire is on the left side engine block, under intakes.

If it is a Syncro, this ground is connected to one of the oil breather
bolts.

The wire for this ground comes out of the wire harness sheath that
contains the wires for the Hall Sender.

Search the recent Archives for discussion on this - it has come up off and
on.

Good luck

Michael
Personal VW web page:
http://www.velocity.net/~vickrail/moshevw.htm