Update.

There are a number of grounds on the firewall that are for various items. Some unknown use as prior owner rewired part of the vehicle with custom items. I cleaned them, the firewall, and added a bond wire to the engine/battery chassis ground, I changed the ground screw to a stainless one for resistance to rust.

I replaced the throttle position sensor. Throttle response off idle is notably improved.
I replaced the cam position sensor. The magnet was visible and felt secure. I inspected the wire lead and connector. The connector had been replaced at some point. Connections appeared to be of quality heat shrink type.
I replaced the crank position sensor. I adjusted the reluctor clearance. But when test rotating the engine with a wrench I heard a reluctor vane ping. I removed the harmonic balancer and used a vernier caliper to guide the tweaking of the vanes. They were likely bent when it was shipped to me last year just wrapped in bubble wrap and in an envelope.
I replaced the O2 sensor.

First start up had near normal idle vacuum. Highest I've ever seen in 30K miles. No idle surge. Test drive though resulted in falling vacuum levels as the engine warmed up. Idle surge returned and Idle was rich again.

Drove 90 miles to an event. Operational vacuum was improved over recent levels but still below accepted norms. Check engine light illuminated occasionally. Fuel fill results show fuel economy typical for the past 30K miles - 12.5 mpg. Idle surges and was rich enough to leave exhaust stains on the ground. Checked ECU codes. 12 and 13.

Return trip included towing an enclosed trailer. Very high drag. I was not popular on the highway. Transmission is equipped with a large external cooler. Normal operation temps do not register on the trans temp gauge which begins display at 150 degrees. I limited speed as 55mph resulted in 225 degree trans temps with air temp of 78 degrees. Check engine light illuminated occasionally at first then no more. Engine performance possibly improved very slightly during the trip.

I checked codes the next day. 12 and 13. I had fitted a wire into the wire harness connector near the ECU for monitoring the O2 sensor voltage 2 years ago. I connected a VOM to that. I drove for 30+ miles. Voltage ranges with 0.9 typical operation with an occasional peak of possibly 0.92v. Idle surges and is rich.

Codes this morning are 12 and 13

Any thoughts?

Off the top of my head:

- Don't tow a high-drag enclosed trailer with this car, even when it is in good shape.
- Code 12: PCM is awake
- Code 13: Oxygen sensor circuit open or no activity

Code 13 would seem to confirm the oxygen sensor . I say chase that sensor and trace the circuit.

The O2 circuit was tested through to the ECM connector. I didn't want to risk damaging the ECM connector by back probing it for testing while in operation. Thus I had fitted a 22 gauge multi strand wire into the connector in the harness only 1ft away. In 50 miles of driving, I have not noticed any interruptions in the O2 sensor voltage stream to my VOM. I'll have to risk the ECM connector integrity by back probing it to check for any issues in the 1ft of wire between the harness connector and the ECM connector terminal.

Yeah, I think you're on the right track. The connector would be what I suspect at this point.