My 2003 P71 has a rough idle, but only when in gear. It's fine when in park or neutral or once i've begun to accelerate. And when i come to a complete stop, it still idles rough.

IDEAS?

First thing I'd try is some Berryman's B12 Total Fuel System Cleanup, 2-3 cans x2 fillups, available at any Autozone or Checker/Shucks/Craigen.

Rough idle can be caused by carbon buildup, and with a P71, I wouldn't doubt it as hard as the cops run them - when I got my 99 P71, it had a rough idle and the mechanic said it "filled the entire shop" with smoke with the carbon removal stuff he used.

I could be wrong, and If that doesn't work, it could be something else mechanical.

I'm sure someone else can chime in but thats the first thing *I* would try.

In my experience it would be a few different things.
One Problem I have been seeing recently with several 2003 P71's (all with over 100K miles) is a Bad Ignition coil. It is a rather easy fix if your check engine light is on. If it is all you need to do is go to your nearest auto parts store to get the trouble codes read which should tell you which cylinder is misfiring. The Coils are about $35-40 each from the parts stores but can be bought on Ebay for about $10. If the code says "Cylinder 1 Misfire" (will display missing cylinder) then that is the coil that needs replacing. Since you say it is not that bad then it is more than likely only one Ignition Coil. Your Car has 8 ignition coils (One for each cylinder) and the coil is attached on top of the spark plugs with only one bolt.
If it is not that then it could be the connections are bad (either the connector that goes to the ignition coil or the ignition coil to the spark plug) so you might want to check the connections first before you start spending money (i have noticed that when water gets into that area it tends to cause the missing by dirtying the ignition coil connections or causing them to fail). If it is not that or the check engine light is not on then it may be something else like dirty fuel injectors.
Hope this Helps!

I've ran seafoam and lucas through my tank, and nothing.

Any new ideas?

How's the IAC? When were the plugs last replaced? Are any of your coils cracked? Have you replaced the air filter? When was the fuel filter last replaced?

Try cleaning the throttle body and MAF sensor, see if it makes a difference.