Well, it turned out to be <drum roll> lack of a ground lead between the
case of the shack computer and the main shack ground bus (a length of 1"
copper plumbing pipe across the back of the operating desk). I caught on
when disconnecting all possible leads from the transceiver to try to
trouble-shoot the problem. The RF was apparently coming in from the
computer to my radio control interface. Anyhow, grounding the case of the
computer made it go away -- things are now stable again.
Thnaks to all who offered suggestions!
73, Pete Smith N4ZR
Sometimes a tower is just a tower
Administrative requests: rfi-REQUEST@contesting.com