HELP: BMW OBD Codes 235, 236

[Wayne's World]

Hi guys - long time! Referencing one of my long threads from about a year ago, it seems my problems have resurfaced. The solution to the problem at that time was an after-cat oxygen sensor.

235 - Pre-Cat Bank 1, Fuel Trim
236 - Pre-Cat Bank 2, Fuel Trim

This time, the symptoms are the same as before:

Idle isn't silky smooth but not anywhere close to rough anyway - I can definitely tell the motor is running by feeling the vibration in the driver's side door panel but otherwise, mostly imperceivable.

MPG doesn't appear affected, averaging 18.5 MPG combined over a period of 1K miles.

Performance doesn't appear affected as it still hauls as much as the 3.0 can.

After two short trips when the motor is cold, the light will be off on the 3rd startup. It's not until I drive for an extended period of time before the light will resurface.

So far I've taken it to my indy and they've done the following:

1. Checked DME code and it is the latest version. There was apparently a service bulletin on this.
2. Done a smoke test on the intake and replaced the air intake boot. Elbow was cracked going into the idle control valve.
3. Replaced the valve cover gasket as it was leaking.
4. Replaced my NGK Iridiums with the NGK originals. Iridiums read as a normal condition leaning towards rich-running. Torqued to 23ft/lbs.
5. Replaced my MAF with a Siemens/VDO one.
6. Replaced the air filter
7. Cleaned throttle body and ICV.
8. Removed and inspected the DISA valve/Manifold adjuster unit.

Oil Separator, spark plugs, fuel filter and pre-cat O2s were replaced roughly 2 years/40K miles ago. In this time I've done a comprehensive 60K service. Since 40K miles ago, I've really done nothing other than the above to treat this issue which cropped up in November. I suspect I ran too strong a dose of fuel system cleaner and this could have screwed up my O2 sensors prematurely (Berryman's B12 Chemtool - 2 cans in 1 tank) and perhaps this in conjunction with the winter-blend/ethanol gas might have just been too much?

I really want to think it's the O2s but hate to fault the O2s for reporting a problem.

My only real inclination is that because of the short trips when the motor is cold, the O2s run a self-heat cycle and perhaps clean themselves enough to run normally whereas once the engine is warmed up, they return to a 'normal' state and therefore are not running optimally?

Any advice would be greatly appreciated. If anyone knows how to test the O2s that would also be greatly appreciated. I've heard that they run between 1 and 1.5 volts DC but can't recall where I read that.