Stephen,

Unless you have already done it I would check the TPS for correct operation. My MC18 300 with Tyga GP pipes nipped/seized 3 times. I replaced one complete cylinder/piston, tried rejetting and many other things to solve it. Eventually it was traced to the TPS which was working intermittently and advancing the ignition too much as full throttle.
Matt will give you the resisitance to check for 8kOhms or something. If in doubt relpace it!!

Why. Because even though the TPS is part of the "electrical" system it is a "mechanical" part and will wear out.

I'm interested in you preference for the 250 motor.

Mike