MP100L Eval current fluxiation

I have a setup that uses the Eval board for the MP100L.
The input is 115VAC at 300HZ. The 12V output is using only 8mA steady state.
But the BK9801 AC Source is showing the input power mostly at 67mA with is jumping to 667mA ever few seconds. Is the AC cap on this being affected by the input frequency?

The input cap isn’t usually going to cause an issue… and the data sheet doesn’t mention a frequency limit. Remember though, this is a 50/60 Hz device, so there might be timing issues.
(and its not guaranteed to operate outside of 50/60 Hz too).
Have you checked your input cap value based upon calculations? typically they are 1/6 the size at 300 Hz

So check you output voltage with an oscilloscope (and an isolating transformer on the PSU), if the voltage slowly ramps down and quickly steps up again, then its more likely output cap or feedback related.
Other check is compare its operation on 50 Hz, see what is different.