A Robinson R22 pilot reported a loss of power and a forced off airport landing. Power loss was later attributed to fouled plug and magneto issue.

Narrative

I was flying around 1;000 FT AGL about to start our initial descent. I noticed that the engine and rotor RPM started to decay with other indications such as left yaw; decreased noise and low frequency vibrations. I immediately lowered collective while simultaneously rolled on throttle. This managed to keep the rotor and engine RPM's in their current position momentarily but didn't last long. I entered a 180 degree autorotation and picked a suitable landing spot into the wind in a large field to my left. During the descent I noticed that the engine RPM holding steady at 55% with the throttle closed. Knowing this I planned on a full down (no power) landing but decided I would try a power recovery in the flare. As I flared I rolled throttle on and power came back. I safely landed the helicopter with no hazard to persons and property. I then advised Tower that we did a precautionary landing and that no assistance was needed. Later I found that the disturbance in the engine was due to some type of fouled plug and possible magneto interference. Two separate problems just so happened to occur at the same time. A proper preflight was done previously and we had no indications of fouled plugs or inoperative magneto's. Because of the training I was given; I was able to process the task at hand and make the decision to land at the closest open area.