TS590 Power fluctuations in Digital Modes

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TS590 Power fluctuations in Digital Modes

I am running fldigi and WSJT-X on my iMac, and they both receive very well, and respond to CAT commands. In the last month or so, after upgrading to Sierra OS X, I have noticed a strange power fluctuation that begins after the first 5 seconds after clicking the tune button. The phenomenon is most pronounced using WSJY-X, although I see it when tuning with fldigi. It does not occur when I send a CW carrier or have the radio set to AM mode. Only digital, and on all bands.

The power dips start very small at about 5 seconds, then quickly increase until power is dropping to zero and back up every 2 seconds or so.

I tried putting ferrite on both ends of the USB cable. Problem persists.
I have tried using a dummy load instead of an antenna. Problem persists.

Here is a short video that I made to demonstrate the problem: https://www.youtube.com/watch?v=E5m8-a-fw6k

Aloha from Kona Hawaii