rp718

Have already replaced a purge valve, and the mass air sensor, but the car still surges (sporadically) in low gear, at low speed, after releasing the accelerator. Typically in stop and go traffic. It's almost as if the idle drops too low, and then shoots back up to compensate. The RPM's surge up and down for maybe 8-10 cycles before the idle finally stabilizes. An M-B tech told me it may be the throttle actuator, but I'd hate to continue fishing for the real culprit, especially since I hear this part is expensive. Any insight?