912 ULS randomly dies at idle

300-400 rpm is usually just a plug or plug wire. The engine run up with a mag check shouldn't die if it's only a 400 rpm drop. So now you need to rule in or out certain common causes. So the first three things I would do would be change plugs. Make sure the thermal paste does not get down on or around the electrode tip. If your using NGK plugs try a gap at .025. That's in the middle of the .023 - .027. If you're using the new Rotax plug leave the gap alone. Unscrew each spark plug cap off each wire. Trim the plug wire end back 3/8" and apply a dab of dielectric grease on the end. Then screw the plug cap all the way back on until it stops.. Make sure each plug cap is straight and secure on each plug. Then do a pneumatic carb sync and see if these things don't clear up that rpm drop.