Hi, My Focus Mk1 1.6 has only one lamda sensor pre-cat. Checked out yesterday on a snap on large diagnostic handbag thing costing way too much & requiring a gaolers size ring off pcb's for all manner of vehicles, simply stated NO faults detected, no codes logged.
I however am having a similar experience whilst in the workshop difficulty in obtaining a smooth steady acceleration through 2500 - 3500 rpm.
Air leak or something I suspect, so I don't think lamda will be your cause. I will be watching this thread to see if anyone can come up with a definitive answer. Just though my problem * having only one lamda sensor * might contribte to a solution by someone.