Reading the thread on the exhaust valve made me think of a magneto problem I had last winter that I thought I would share. The problem happened While on a trip from Brownwood, TX to Durango, Co in January with 3 passengers. I was on an IFR flight plan but in VMC. In order to go direct and save some time we elected to climb to 15,000 somewhere south of Santa Fe, NM. Shortly after leveling at 15 the engine began to miss fire and shake violently. So much so we could not maintain altitude and were in a 3-500 FPM descent. We were probably about 15-20 miles southwest of Santa Fe. I located a small airport about 10 miles east of our position. I can't remember the name of the airport. We told ATC we where heading there and we could not maintain altitude. A quick call to the Unicom revealed that they had no maintenance. As we we descended the engine smoothed slightly. I advised ATC that we wanted to try to make it to Santa Fe. Meanwhile I attempted to trouble shoot the problem fuel, mixture, temperatures, pressure, etc. My big mistake was not backing up my actions with the check list. Being over come by events I never thought to switch the mags to L an R. If i would have used the check list I would have discovered the problem and probably smoothed out considerably. By the time we were on final approach the engine sounded normal. Once on the ground and clear of the runway I did a mag check and they checked normal. Santa Fe Aero services pulled the data from my JPI EDM 930 and inspected my magnetos they revealed the problem. One of my magnetos was leaking oil and cross firing at altitude. If I remember right the mechanic told me that my block or rotor button was the old style without the grooves and the leaking oil because of the bad seals was causing the cylinders to fire at the wrong time. He said if I had the type with the grooves it may not have happened. We decided to rent a car and continue on our snow ski trip to Durango and Telluride. We left the plane in Santa Fe while they replaced the seals in the Bendix magneto. After returning home my A&P sent the magneto out for overhaul.