A C172 pilot reported a RPM loss and a rough running engine after the aircraft reached its 2;900 FT cruise altitude. The aircraft was returned to the departure airport.

Narrative

We took off for the morning traffic watch flight. We reached cruising altitude (2;900 FT) about 7 minutes after. I proceeded with the cruise flight check list. 2 minutes in to cruise flight we lost about 400 RPMs and the aircraft started shaking very hard. We turned back around toward the departure airport. I had to go full power to maintain cruising altitude and the engine wouldn't go over 1;800 RPMs; when in normal operational conditions at full power the RPM's would be at around 2;400. I thought it was vapor lock. The airplane is fuel injected and it is normal to vapor lock during hot summer days; but it is not known of happening during cruise flight; it only happens during engine starting. Just in case it was vapor lock I tried fixing the problem by turning the electrical pump on and also by gradually increasing the mixture to full. Turning the fuel pump on did not fix the problem. We kept going in for landing and as I decreased the RPM to initiate the descent. The shaking of the engine was even worst than before; we landed and taxied back to the parking ramp; while on the ground the airplane was still shaking and the power was very limited. The airplane is being checked by the mechanics right now; at the moment I have not being informed of what the cause of the problem is. I spoke to an experienced mechanic before they took the airplane in to check it and he said that he thinks it is a broken cylinder. He also said that a bad magneto does not make the airplane shake and also said that vapor locks are not known of happening in cruise flight with a mixture leaned. Also it could be a stuck valve; but the engine has only 480 hours of operation and that should not happen to such a new engine.