I had a similar situation in my 85.  Driving down the road, if I eased back
on the accelerator, even a little, the engine would cut out.  Turn the key
off then on, engine fires right back up.  I got very adept at either powering
through corners, or turning the steering wheel and the key at the same time.

The fix ended up being a new ECU.  Got one from a yard for $70.  See if you
can borrow one from someone and swap it to see if it works.

Also, as always, check every ground!  ALL OF THEM!!!

Bill Marshall
85 GL Tiico "Pandora's Box"
Bradenton FL