How good are you at recognizing abnormal O2 sensor beavior

[GRUMPYNOMO at aol.com]

Iam befuddled!  I have a Range Rover with O2 sensors that drift downwards in
voltage.  Just warmed up the sensors cycle from 800mv to 0mv.  As the engine
gets to operating temperature the cycling range drops to 600mv (hi) to minus
300mv. (low) with a mean of around 100mv sometimes even below 0!

What's wrong with this picture?  What could be causing this?

I have added an extra ground to the sensors and I have changed both of them
and I have substituted another ecm.

I am going to unplug the alternator and retest.  Please give me the benefit of
your opinion if you have a theory or if you have experience in this area

Thanks
Alan Briggs
FCECLtd at aol.com