The ol' girl is an '89. The harnesses are, in my opinion, not the best looking. I know one plug has one wire that is bypassing the plug itself(the plug that connects the back harness to the front harness). That was how it was when I got it. As for grounds, they all seem okay, but I can go over them again.
SCs live/die by their electrics/electronics. A compromised harness can include visual damage as well as hidden. Some wires are shielded to fight off EMI..electrical interference in signals that can confuse the EEC. When the shielding is damaged/missing, funny things can happen and be difficult to identify/isolate. When pins are loose/corroded, signals can drop out entirely.
Grounds are part of the same environment, making it hard for reliable signals to come from and go to the EEC and all those sensors. Inspect the cable on the pass. side motor mount that goes over to the body. Corrosion can hide inside the cable sheath and where each end is bolted up. Best to install a new one there, just don't use one longer than needed. Be sure the metal and bolt hole on the body are clean shiny. Replace any sad looking battery cables as well.
Confirm the ground wire at the back of the head where the fuel-pressure regulator bolts up is also clean/tight.
The mounting surface where the DIS sits needs to be bare metal, no paint (
yes, you still use the paste that a new DIS comes with in the box)...same where that entire accy. bracket bolts to the head/engine. Painting those components can hamper the ground path back to the battery and cause signal issues.
Be sure the voltage at the battery is 11.5 or higher, both engine off and running, where it could be as high as 14+ volts. With the key off and the headlights on, check to see what volts are present across the battery terminals...keep checking for a minute or so to confirm it's ok, or if it drops like a rock.
All this can be tedious, i know, but an older/neglected SC is a prime example of hard to find niggling electrical issues....pretty sure that's why I was able to buy mine - I think the previous owner got tired of not being able to figure out why it would run ok one day and act up the next