[AndrewCologne]

Originally Posted by 801coppertop

Here is the issue as best as I can define/pinpoint it.
Engine runs well as it is warming up. Even into closed loop (a bit of oscillating idle but runs ok)
Once engine is fully warm/heat soaked engine runs lean, starts to backfire a bit off idle.
Runs well at speed and under load, with the occasional stumble.

Connect a DMM with duty cycle option to the white/green wire (pin/terminal 17 of the ECU). Red connector of the DMM to GND and the back connector of the DMM to that green/white wire. Connection here is inverse cause the duty cycle of the ECU affects the GND connection of the frequency valve.
Now check all states when driving where you say that the mixture leans out ... and report here.

Also have a look here at the 930/16 engine workshop for proper initial setup and for solutions on issues with the lambda system:
https://nineelevenheaven.wordpress.com/the-911-sc-3-0-engine-with-catalyst-and-lambda-control/
https://nineelevenheaven.wordpress.com/the-lambda-control-of-the-911-sc/