We have a 2004 T206H and have 60% success in starting this engine on the first try using Cessna's procedure in the POH, this has also been the success rate using 4 other procedures that we have been given by other pilots. We have had the engine checked out by the Cessna service center in SLC and it checked out fine. They had suspected an ignition problem but could find no problem, and of course it started fine for them, which it usually does after it has been started the first time, it took us 5 tries to start it that morning. The acft is hangered and kept at 50 degrees, so cold wx is not the problem. Also during some prop checks it will start running rough, as if a mag has failed, but will clear out as you increase the throttle back to 1800 rpm. Thats where Cessna suspected the impluse coupling on the left mag, but it was fine. Any suggestions?