I have the identical problem with a 1999 Maxima. Drives ok on highway, but
in city driving or pulling hills, drives you crazy with the missing, rough
running and loss of power.
New plugs, etc. did not help.  When I had it in for service, was told (and
shown) sludge had formed in the oil system, obstructing oil channels.  It
was cleaned up, ran ok for a while.  Now doing it again.  I was told to
use an oil for older cars, one with higher detergent content.  Have not
tried it yet, but car is getting worse, time to do something or take it to
another mechanic who knows what's wrong.

My personal theory.  Valve guides are sludged up.  Does not stick when
cold, but on warming up, valves stick in guides, not closing properly.  

Do they still make Bardahl?

My next stop will be the auto parts store for some free advice, oil or
fuel additives. If cheap solutions do not help, next stop after that,
another mechanic.

If you have found out anything since your least posting please let me
know.