Hi Jeff,
Thanks for the reply.
Some answers to your questions. Yes each major piece is grounded to a common ground separately . I have and RSP DUo and 2 TX/RX switches that are not grounded separately, my next test I'll remove them and run direct to antenna. from the tuner.
To bypass the 998 you can manually bypass it so it routes directly to the antenna (or power it down). I do this when I'm on a resonant spot on the 80m dipole 3.752 mhz where the SWR is 1:1 and a tuner is not needed. When transmitting 100W from my Yeasu ft-991 I don't have the same apparent RF interaction. I have tried using an SDR to see if there is a resonant Frequency that appears only when using the 998, but it wasn't obvious if it's there.
Interestingly enough I replaced an MFJ-962 with the 998 and I don't recall having this issue (after I installed some RF chokes on the lights and Oven)
I do find the 998 so much easier to use than the 962 and hope I can isolate the issue, meanwhile I refrain from the 80 m band
73 Mick VA3EPM
Mick