I was flying KFTW to KELP today in IMC at 10,000' at 40 deg F. Tail winds were good (168kts over the ground!) so we changed our destination to KDMN. About 8-10 miles northwest of KELP, I noticed the plane slowing a bit, then cylinder #5 went cold. It slowly dropped to about 230 CHT and 1000 EGT, then much lower than even that. We declared an emergency and headed to KELP. We broke out the the clouds just west of the mountain ridge west of the field and found a nice gap to go through. It was running ok and we had to make a few S-turns to get down to 8R. As I got lower it felt like it ran worse at low power so I kept the power in and told tower that it might quit on the runway. I make a perfect landing and on roll-out, the engine did quit. I didn't really have time to troubleshoot things and just wanted to be out of IMC and on the ground. It may have quit due to too lean mixture but I don't think so - otherwise more power on approach should have made it worse. A local mechanic started it and it ran ok. He removed all 12 plugs and said the ones on #5 were not working at high power but cleaned them and now they work. I don't think I was running overly rich... 67% power and about 13.1gph. All cylinders checked out at 75 or 76/80. Exhaust looks fine. Dunno - bit of a mystery. The mechanic and I took it two laps around the pattern here. Any ideas?