MAF puzzle

[j.fro]

A few years ago I got an MAF CEL on my 2000 986S, so I replaced the original 124 MAF with the updated 125.01. I took the car to a shop who said they could do the ECU update. After that, the car ran fine, no CEL.... but when I dynoed the car, it showed a lean condition, around 16:1, at idle and part throttle. Under full throttle it richened up.
I've been using Durametric to try to figure things out. With a new 125.01 MAF, both O2 sensors read 0.09 volts and the MAF shows 18-20KG at idle. Under throttle, the O2 sensors may bump up to 0.2, but not much higher. Looks pretty lean to me.
I tied the 124 MAF for kicks. It also reads 10-20KG, but the O2 sensors read 0.8 at idle (looks rich to me), then drop to nothing under revs...and the engine stumbles.

Anybody else seen this?
The O2 sensors have about 25K on them and the car has a Sotronic reflash.
I've tried unhooking the MAF altogether and MAF reads 0, the O2 sensors read 0.05 and drop when revved, no stumble in the revs, runs great.
From what I understand, at 14.7:1, the O2 sensors should read 0.45