1993 Audi 80 2.0 single point (ABT engine).

It's my dad's car, had it from new & been well serviced over the years. I'm
trying to diagnose poor top end performance and thinking it may be the
lambda sensor, or any other suggestions welcome.

Starts & drives well but gutless at top end. Up to about half throttle it
has plenty of pull but put your foot down for overtake or approaching
inclines and there isn't much there and takes a long time to build up
speed - usually have to knock it down the gears. Feels as though it wants
to go but a lack of fuelling at top end is holding it back. In fact if your
cruising along say at half throttle and plant your foot down it feels like
it actually slows down before very slowly starts to build up speed. TPS &
various other sensors appear ok (within Autodata spec) & checked for any
vacuum, air leaks. The injector unit to manifold rubber flange looked a bit
worn & cracked but I've replaced that.

Only thing I can think of is the lambda sensor as the cars done about 120k
miles on the original so was wondering if that could be on its way out and
slow at sending the correct signal to increase fuelling at top end?