upper rpm break up

Having some issues with my daughters dart, we took the car to a test and tune and in drive towards the end of the 1/4 it acts like its hitting the rev limit, msd 6al2 so I turned the limit way up thinking maybe the dial was off but still the same. I tried a different pump thinking maybe it was starving but no luck. Its a mild 440, rpm heads 10.5 comp. and a hughes hy. mid sized cam. On the last pass on the return road I could smell something hot (electrical smell) and the msd box was pretty hot, thinking maybe the box is the problem? runs great in 1st and 2nd all the way to 6300 shift but in 3rd holding high rpms at the end is when this happens. Any ideas??