i say it's classic moosing. My car used to do that as well. If your idlign the car and turn the wheel, the computer bumps up the idle a bit to compensate. If the IAC valve is sticking or whatever, the car will moose. Change the IAC.

1999 Silver Frost SVT #609 of 2760 Quaife, lightened SVT Flywheel, SPEC stage II clutch, removed resonator, k&n drop in - various other goodies too.