The grey plug on the firewall is one of two places to see the condition of the wires in question. The other place is at the alternator.

There is a harness that runs under the front of the engine. It contains three wires. One is a red very heavy gauge alternator to starter, the other two are 16 gauge or so. Red to the alternator, black to the oil pressure sensor. That harness fails after 15 or 20 years, with symptoms of (1) warning lights coming on while car is running, (2) oil pressure light coming on, or (3) if the heavy wire gets shorted, fire or at least a lot of smoke and smell.

The total problem is that the wiring inside the large fat harness running along under the intake manifold goes bad, too. Heat, brittleness, and I don't know what-all, and the wires in there can start to short to each other. One such wire "talks" to the ignition coil.

Your symptoms sound like intermittent ignition failure.

Contact Dave Barton, a brickboarder, to get a new harness. It will go from the ECU (in the cabin) to basically everything on the LH side of the engine, and the three under the engine. Feeds the injectors, too. It will include the wires out of the grey connector, I do believe.

Wiggling the main harness can make the situation much worse. A good idea to have the replacement on hand first.

Good Luck,

Bob

:>)