Folks,

I have 1991 Maxima SE 5-speed with 160,000 miles on it.  I've owned
the car from day one and it has been rock solid everyday.  However,
I'm having a strange problem with the idle.  Intermittently the idle
will rise and fall between about 500 RPMs to 1300 RPMs.  Eventually it
will settle down to about 750 RPMs and runs smooth.  When it gets down
to the 500 RPM range, you can feel it that its about to cough and
sputter but the engine will start to rev back up so no stalling
occurs.  Running the A/C naturally tends to cause the problem to occur
more frequently.

I'm not sure how to go about diagnosing the problem.  I've checked
plugs, wire, cap, and rotor.  All are good but I did discover one wire
wasn't securely fasten to the cap which gave me hope that I had found
the problem.  The problem occurred again while at a drive-through
during lunch.  Afterwards I checked the wires again and they were all
secure.

Air filter is new and the gas filter was last change around 100,000
miles.  However, my gut tells me it something electrical.  I say this
because I think I started to notice the problem shortly after changing
the alternator.  The original alternator died and I replaced it with
one from an auto electric shop which apparently had something wrong
with it too since it caused me to be left on the side of the road a
day after replacing it.  Because I had to have the car early the next
day, I couldn't wait for the auto electric shop to fix the rebuilt
alternator and had to settle for an el-cheap-o from Auto Zone.  Could
this possibly cause the problem or could the failing original
alternator have caused an electrical component to fail?

One last thing and I know I'm reaching but thought I would provide as
much information as possible.  Since the price of gas has gone up so
much, I've been using the 85 octane gasoline as oppose to the 87
octane and 93 octane gasoline that I normally have used through out
the life of this car.

Thanks for time and help.
rick.