looping idle

[phantastic1]

running into a problem w/ a '92 ls400 w/ a looping idle. doesn't matter about the temp of the engine...cold/warm...it jumps between 1500-2000 only in park or neutral....anything else...it will idle just fine. given it a tune up already...plugs, wire, cap, rotor, and air filter. pcv was still good. cooling system is free of air...so i dont thats an issue. couldn't find anything online about looping idle for this car. any help would be appreciated.

[Stealth91]

o2 sensors is what i am going to say for this. In our shop we had a 94(i think) explorer which would idle around 3500 rpm. it got 1/12 of a mile per gallon. Couple of theoretical problems here
1. o2 sensors malfunctioning
2. Mass airflow sensor is telling ECU more air then there actually is
3. Injectors stuck open
4. Fuel pump pressurizing the fuel way to much
5. (we found this out by accident) somehow gasoline was leaking into the oil system. gasoline is nowhere near as good of a lubricant, so the enigne had to idle higher to overcome the increased friction from a horrible lubricant. open up your oil cap/dipstick and smell it. If it smells like gasoline, your in trouble.

But, im still placing my money on the o2 sensors or the mass airflow sensor