Your ignition relay is shorting out. One of the contacts is shorting against the metal case of the relay. For some reason the relay expands just enough to where there is a short. Happened on our 87 Jettas (we had 2) and especially dangerous on the freeway driving 70, middlelanes. .
Relay on our 87Jetta was in the cabin, under the dash, left of steering, Square metal rectangle, It is a plug in part- was a cheap part. GL
LP.