Guys if this is not happening when idling in neutral or when stopped in drive and is only happening when the car is coasting then its your torque converter. Check fluid level in the tranny...if it is properly filled then if you want to fix this issue then the torque converter has to changed.
Basically when the engine is not under load and the car is allowed to coast the wheels are freely spinning and the driveshaft is moving faster than the engine flywheel. The torque converter is responsible for doing torque manipulation to counteract that speed difference and keep the engine RPM constant. When the torque converter is not working right it will not do its job properly and therefore you will have the idle fluctuate.
Last edited by DFEL750I; 11-15-2012 at 06:42 PM.