Just thought I might chime in. My 250 I just rebuilt had a similar problem on the top end and i thouht it wad a jetting problem. At the end of the ride I turned it off then it wouldn't start. Thought it was a ignition coil. Tested everything and found it was the primary coil going bad in the stator. I know you replaced it with another but is that a new one or a known good part. Good luck with your problem. Hope this helps.