99 Sentra weird mid-rpms issue

[Codifus]

My car's been acting strange of late. On the rpm gage, from idle to
about 2500 rpms it feels fine, quite strong actualy. But at about
2700-3000 it gets "confused." it accelerates to 2950 rpms, then backs
off to 2900, then accelerates etc. All the while my foot is being held
steady on the accelerator pedal. I have to floor the car to downshift to
a lower gear which would put the rpms past about 3500 rpms. The car
would still feel a bit funny but less so. As the rpms continue to rise
the engine seems to gets its strength back. At 5000 rpms it feels
completely normal and from 6000 to 7000 (redline) its actually producing
some decent accelerative motion.

What would cause the mid-level confusion? Timing? MAF going bad but not
quite? Fuel injectors?

The effect is very pronounced on those long mildly sloping uphill roads.
The car gets so weak that I have to put myself in the slow lane to let
everyone go by. I know its not the strongest car to begin with, but its
getting a bit out of hand.

Thanks

CD

[cod...@optonline.net]

OK found the problem, 2 things actually.
O2 sensor and timing. Once I replaced/fixed those my car came back to
life. In all the cars I've owned this is the 1st time I've replaced an
O2 sensor. I've always been of the belief that if the O2 sensor still
works, don't replace it.

CD