A C172 lost power at takeoff RPM but at reduced RPM the engine developed enough power for the pilot to return to the departure airport from 900 FT.

Narrative

Aircraft: 2007 C-172 G-1000 GFC-700. Weather: Ceiling and Visibility unlimited; SKC; Wind 310/15. A friend and I departed on Runway 31 for a left VFR departure. On the upwind as we climbed through approximately 400 FT; I put the autopilot into a VY climb in heading mode aligned with the runway. Climbing through 900 FT; I heard the engine hesitate a couple of times and then begin shaking. It wasn't particularly violent; but it was enough to be clearly unusual and something had obviously gone suddenly wrong. It was no longer developing full power. I immediately disconnected the autopilot and began reducing power and turning to return to the runway (the emergency landing options were better in that direction even if I didn't make the runway). I notified the Tower that we were returning with some form of engine problem. I found that the engine smoothed out noticeably around 1;500 RPM. Good news. We still had partial power available if necessary. I found myself very high and in no position to land downwind; so I decided to try and use the partial power available to land back on the original departure Runway 31. I had already completed the teardrop turn to get lined up with 13; so I had to make another teardrop turn for 31. Tower had everyone out of my way and I communicated my intentions throughout. I flew a very tight close-in teardrop and rolled out on short final on the VASI at about 300 FT AGL. Not a maneuver I'd want to try normally; but; in this case; I really believe it was the safest and most effective option. If the engine had not given me just a little power in the turn; I would have had to roll out and take an emergency landing either on an adjacent road; or into the mud flats. But I had enough power that I thought it worth the risk to try and make the runway. It worked out; we made a smooth landing back on Runway 31 without incident. At low RPM; the engine was smooth enough to taxi to parking without assistance.