slight stumble with factory 402

[2500HD]

I am looking for help with a slight stumble from 5 to 9 mph and then again from 25 to 31 mph. It is very slight with light throttle and more prevelent with medium throttle. I first thought it was a bad carrier bearing but have ruled that out after inspection and new exhaust was installed.

To cure I installed a pertonix and removed the points. At the same time I installed new cap, rotor, coil, plugs and plug wires. I did not think my vacuum advance was working so I installed a new one after the first test drive and it did not solve the issue. Plugs are gapped at .035 and there is 12 volts to the coild and the pertonix. Timing is set to 9 or 10 degrees. It starts quick and idles smooth.

Any ideas would be appreciated.