You were fine with the stator cover removed. I believe it was just to cool the oil pump drive bearings. When it does this, have you reached down to feel the water temp ? It is poss the by removing the choke, it created a slight lean issue, so I would give the high speed jets a little more out. Maybe 1/16 of a turn, but that wont fix your issue.
Have you checked spark on all 3 when this happens ? Could be a bad plug wire. Might try trimming them back about 1/2" and see if that helps.