Another update... I was able to add a 3 wire O2 sensor to the car. I used a Bosch 13918 which was powered with a relay. I used a positive signal from the unused thermo valve plug that I mentioned above and constant 12V came from the Heater fan fuse in the rear fuse panel. Unfortunately this made little to no difference in the surging effect while driving with light loads.
I also spoke with the folks at Systems Consulting regarding the ECU box. They mentioned that these Lambda boxes are generally pretty reliable and generally work or don't. I described the problem I was experiencing and they didn't feel that the ECU would be to blame but did offer to test it for me.
https://systemsc.com/index.htm
I am still on the hunt for the next thing to check