Just throwing stuff at the wall,

Have you replaced or repaired the Engine Harness by chance? I know there are lots of postings about strange harness related issues in the Pre 87 or 88 cars. 85 and 86 are often mentioned having problems with crumbling wiring.

Wondering if issues in the harness where it goes through the 3 and 4 gap in the intake manifold or in the smaller one were it goes under the engine could cause this sort of problem?

Now to call my own wall toss idea into question. Stumbled into a genuine Volvo replacement harness in an 85 down at the PUP and gleefully installed both into my 86. The four wires on the big connector were crumbled and as we say in Texas had lots of necked(naked)copper stands exposed. Wiring going to some of the under the hood lighting and all of the washer motor connections were a mess. Those issues along with that lots of BB postings on 86's with bad harness, lead me to believe the whole harness was going bad. A couple of months after transplant surgery, decided to do a postmortem on the two old greasy stiff harness that came out (before tossing). Sliced off the old stiff black tubing at the intake manifold bend and several other places. Found the only issues were the wires from the gray connector on the firewall. The large harness and the rest of the small harness wires did not have a single wire damaged or even close to having a problem. The smaller harness may have been replaced by the dealer, found a cryptic reference about replacing distributer signal harness on a dealer service invoice in the file. The previous owner (my Mom) does not remember what that was about. Guess at some point before 88 better wires were installed in at least some of the harness used.

Might be worth crossing it off your problem check list if there are insulation issues in your main harness.

Regards,

Paul