Originally Posted by Technical Ted
I think the PCM ground is under the ICM mounting bracket on the 89s. I'd check the battery cables & ground bus too.
The wiring harness that carries the Vehicle Speed Sensor and oil pressure sender comes up from the passenger side rear of the motor (behind the power steering pump) to the steel lift ring.
The wiring harness for the ignition module, A/C, and a few other odds and ends
crosses over the manifold on the passenger side of the motor and joins up with the harness from the VSS & Oil pressure sender...then the whole bundle jumps the gap from the engine to the body on it'* way to the ECM.
Sooo... that would also be a place to look..harness wires grounding out on the block or some metal bracket on the motor. Those wires were all sheathed in black plastic wrapping, but the car is pushing 20 years old and it'* possible that the protection is breaking down.
I'm betting it'* just corroded contacts in connectors/ground wires, but that it comes and goes with vibration sounds like something shorting out on a part of the motor or body.