The A/F meter should be hooked to it's own sensor as per
instructions .
If it's hooked to the same O2 as the computer, this may be causing a problem.
Check the fuel pressure if in doubt. The pump, filter, and FPR can cause problems.
High RPM power loss can be the first sign of a CAT problem, IMRC will manifest itself around 3800 rpm.
Is the rear O2 as new as the front?