Low RPM. Headlights have a dedicated circuit that get more power at higher RPM. Now did you replace it with the bulb recommended from the manual? The light may be drawing all the amps the bike is putting out. This could be more than the bike can handle.
Try disconnecting the tail light and see if it helps.
I just wanna go fast. If your not first, your last!!
Reproducing the Tecate CDI. Contact me if you need one. I'm most accessible on FaceBook. You can find me on the 1984-1987 Kawasaki Tecate KXT250 Group.