odd idle problem

my 96 3.5L will idle great when it starts, but gets a little rough after it has been driven even a few feet in drive. If I pop it into neutral and then into drive, the idle smooths out until the car moves again. Does this sound like an Idle Air Control Valve problem, a Throttle Position Sensor problem, or a Tranny problem??? Or maybe something else?

well, try turning your ignition key on/off 3xs ending in the on position and read out the number of times the check engine light flashes. If it flashes twice, pauses then flashes another four times, that's a code 24. If you get no other codes than a code 55, than you have no codes at all. The next thing is when was the last time you had a tune-up?

I've read it out several times, but all I got was 55's - except after it stalled one time, and that got me a 14 (MAP sensor). Am gonna check that out tomorrow. Also gonna check voltages on TPS. IAC valve seems to be working. Car wants to idle at 800 in park and 550 in drive, give or take a little, even on cold mornings - is that normal? No info in the Haynes manual.

Just got this car a month ago. I replaced plugs, air filter, PCV valve immediately, just on general principles. Plug wires are pretty new, I think.

idle @ 800 in park=normal
idle @550 in gear=normal

The next thing you should try is taking off the plastic snorkel that goes to the throttle valve and clean it out for any carbon or other junk that would get clogged up in there.

Took a look at the snorkels and the throttle bodies too - all clean. There is some crud in the intake plenum, but I couldn't see too well with a mirror.

Also, vacuum gauge says 19 in-HG steady at idle, drops to zip and returns correctly when I snap the throttle. Can't find any vacuum leaks (checked w/ propane torch).

The coil pack and plug wires check out okay w/ volt/ohm meter. Didn't get to check sensors yet - too cold to stay outside.

Unless you have any other ideas, and if the sensors all check out, I'm thinkin' it might just be some crud in the intakes.

Thanks for the suggestions.

I experienced something similar one time working on a '95 Eagle Vision, 3.5. It acted the same exact way as your car. Come to find out it was the TPS that was acting screwy, darn thing was showing for feedback voltage 1.5V at idle when it was supposed to be at 0.6V. It never hurts to check that as well as your reference voltage, make sure the PCM is sending the right 5V reference to your sensors.

Checked TPS and MAP sensor - ref and other voltages okay.

Last time a car did this to me ('74 Nova) it had a bad rivet in the distributor bottom plate that was letting the points wander intermittently. I realize this car has no distibutor, but since the coil pack checks out, I'm gonna put in good wires (these look like cheap Autozone wires) and better plugs (I put in the $1.19ea Bosch ones, might get the double platinums or Splitfires).

In addition to the roughness in gear, it seems to break down just a little, maybe 25 rpm every now and then, in neutral. Could be leaky wires.