Thanks for the reply and for all of the info. Now I have some more questions.
1) add a feedline choke to the coax: close to the feedpoint, at the shack end,
or both. (Or more, if needed.)
I have a feedline choke between the tuner and my entrance panel. Doesn't the balun at the feedpoint eliminate the need for a choke at the feedpoint? I thought it forced equal currents on both ends of the dipole thus minimizing or eliminating common mode current?
2) change the length of the feedline by 20' or so - that will change the current
distribution along the outside of the coax and may reduce the common mode
current on some bands. Might increase it on other bands, of course.
I could make the feedline longer, not very easily, but not shorter. Since that sounds like it could effect other bands, and is a bit of a science experiment, I probably wouldn't do that unless I had no other options.
3) Connect a quarter wave radial wire for the problem bands and run it around
the floorboards out of the way. This also shifts the current distribution and forces
the rig to be at a low impedance point for those bands.
Where do I connect a quarter wave radial, right to the tuner ground or somewhere else?
4) Check that the mic leads are appropriately bypassed where they enter the rig.
Ferrite beads on the leads may help, too. You might be able to wrap a few turns
of the mic cord through a ferrite core as well.
5) Make sure that he headset uses shielded wire for the mic, and that the shield
is grounded to the proper pin. Some mic plugs have separate DC and Audio ground
pins, and grounding the shield to the wrong one can make the mic more susceptible
to RF ingress.
Quite possible that the mic lead isn't shielded properly, or some other difference
that allows the RF to get into the mic circuit. (I've also seen this problem when
someone was using a rear-panel connector for audio from a sound card.)
What does it mean for a mic lead to be bypassed, how do I check for that? I'm also not sure how to check for shielded wire or where it is grounded to without cutting open the cord. I'll try my stock mic to see what difference that makes also.
6) Switch to an antenna that isn't a prone to common mode current.
That is what I get for putting up a jack of all trades antenna. I wanted one, all band antenna that would work ok on the low bands so this is what I ended up with for now. I don't need perfection, just useable.
Thanks again and 73