When I let off the throttle in neutral, RPMs REALLY drop
 

In neutral, when I rev the motor, then let off the throttle, RPMs REALLY drop, car almost stalls. What could the problem be? I have a freshly cleaned air filter.

Vaccume leak(try spraying a combustible fluid over all charge piping and check for a change in rpms), a stuck IAC valve (idle air control valve, try cleaning it. Its the valve on the underside of the throttle body), or you need to jump TEN and GND and set the idle.

Im about to pass out right now, but if noone else helps you out by tomarrow night ill link you a few pages that might help.

Im in the same boat, and have been for quite a few months =/

Did you unplug your ecu....I find sometimes it takes about a week for my idle to find itself. Also, if the flapper door on your afm isn't closing all the way, it will leak in unmetered air (this can happen if the afm flapper door adjustment has been goofed with, or the afm is tilted on its side).

If I adjust the idle speed, does that in any way affect my timing?

Neutral switch on the transmission or clutch switch on the ... uh clutch. I'd guess clutch switch.

vac leak? lightened flywheel?

NO...and that's where you should start, adjusting your idle, just make sure you jump ten and grd, if you don't you wont adjust your idle right.

anything done to the motor performance-wise?

Bunch of stuff to the motor, thicker headgasket, arp headstuds, full exhaust, greddy turbo, Bipes ACU, 1.8 injectors, 1.8 flywheel, 1.8 clutch, etc.

vaccum leak for sure does it only do this when cold?

Does it do it when you are in gear with the clutch pedal pressed as well as when you are in neutral (out of gear and off the clutch)? I would check for a switch problem.

BTW, mine does just the opposite (rpms go up by 1000 rpm). I know I have a bad switch but I am too lazy to replace it <G>.

do you have AFR gauge. it could also be your motor leaning out

alternator draw perhaps?

id look for a vac leak first. there should be no reason to adjust your idle