My first guess would be RF back into the ubitx. Especially since it is with the linear amp on.
Do you have the ubitx in a metal case, is it grounded, do you have a common mode choke on the coax to the antenna, and maybe a snap-on ferrite on the line to the amp?
Does CW have the same issue? If not, then you could be overdriving the audio causing splatter which will feedback into the Nano control lines.
I would start by making sure the case of the ubitx is grounded. Then I would do the CW test and verify that it is not overdriven audio. If the problem is still there, then I would start looking for a common-mode current reduction on the coax lines. For me at lower power snap-on ferrites around the coax have worked. At the higher power, you may need to make an ugly current balun.
The above are just suggestions. Your situation may not be the same as the ones that can cause the above, though it is most likely.