Ken,
My TS-820S has a similar problem, but your symptoms are worse.
With mine, the problem is in the VFO varible cao. The grease
(conductive?) on the shaft has dried out and caused the tuning to
become intermittent. Since this is a rig I don't use much, I haven't
torn it apart, but others have and cleaning off the old grease solve
the problem. Try tuning running the VFO from one stop to another
a few times and see if the problem goes away (it does on my rig)
or gets better.

Steve W6SSP
p.s.-my recently aquired TS-520S is just starting to show the same symptoms,
oh joy!