cr 250 ignition problems

just rebulit my cr 250 and it runs good until 1/2 throttle then its spark goes crazy.it starts first kick idles and wil have good power til 1/2 throttle.i hooked a timing light to it and it was showing good spark until i held the throttle steadily at 1/2 throttle then the light goes dim and blinks a little bit. i was wonderin if anyone has any suggestions what it could be thats causing the problem.
thanks jerrod