Fluctuating idle on 1995 Mercury Tracer Trio 1.9L?

Idling on my 1995 Mercury Tracer Trio (1.9L) recently started fluctuating wildly when I shift the automatic transmission into REVERSE. When reversing, you can feel the car surging slightly, like a bunny hop. I also notice the idle fluctuates a little also when in DRIVE but stopped (eg. foot on the brake at a traffic light) but nowhere near as bad as when in REVERSE. If I shift into NEUTRAL or PARK while waiting at the traffic light, the car idles normal again. Any idea what could be causing this?