My 912 ULS was new in 2001 and was never even out of its plastic sealed bag until a year ago when I first mounted it in the plane. All rubber and metal surfaces appeared like new condition with rubber components still pliable and no signs of age. During recent first starts at my home's shop it started quickly and idled great. At my shop I only ran it between idle and mid range rpm, both ran smooth. Oil pressure and temps have all been well within normal ranges. I very recently brought my new finished plane out to my hanger to do its first taxi runs prior to first flight. As before idle and mid range rpm operations were smooth and uneventful. During the initial taxi run while bringing the throttle down from midrange rpm down to idle (2000rpm) the engine died suddenly. The engine restarted fine with a little choke and I thought it was just a random event. However, a mag check at 3500 rpm showed a drop of about 500 rpm on one side. After taxiing uneventfully for another 15 minutes with random throttle settings at idle I thought everything was fine then it died again while bringing the throttle back to idle from mid range rpm. After that occurrence it turned over fine but didn't want to start right away, seemed like it was firing not on all cylinders. Rather than over stress the starter I pulled it back to the hanger to see if anything obvious presented itself. I pulled the carb bowls and they were both full of gas. Any thoughts on what I should look for? Any ideas are appreciated.