Duude!
There I was, thinking you'd have some 21st Century JohnnyCab of a Passat, or something, and all the time, you're driving one half a decade older than mine!
Not going to be engine management, then.
Have now found this -
http://www.ask-knappy.com/knappy/Car%20Stuff/67.pdf - which appears to apply to all VWs from '83-'92.
From those diagrams, it looks like it's the emission control system. So, not my first guess at lighting-bolt-related at all.
Good thing I'm not a mechanic. "Hmm, everything looks ok. Ah, there's your problem, not enough lightning bolts. Try raping and pillaging in the name of Thor for a few days and it'll fix itself, sir."
Then again...