Needless to say it performs perfectly on the bench, so I'm no further into finding out the cause of my problem. I did check out the different options for Lambda sensors, and while there are four options for 5v sensors, the 0-5v (+) appears identical to the 5-0v (-), and the same with the 0-5v (-) and 5-0v (+).
Next step is going to be breaking into the wiring harnesses and seeing if there's any bad connections. With the TEC97 box removed, I did put the 'scope probe on the Lambda signal pin of it's connector, and it appeared to be OK there.
BTW I also tried the Polish TEC99 software, and it seems to work fine on a TEC97 box, so an English language version would be gratefully received.

I also tried the TEC99 EVO software but it wouldn't connect, The only have an Italian version, so I have no idea if the error message indicated a COM port problem, or if it's not compatible.