It seems to be a problem with intermittent contact in antenna relay or
attenuator, so I wonder if somebody has some experience with softening
voltage for switches and relays?
The relay and attenuator for FT-901/902 is shown on
http://home.online.no/~la8ak/93a.htm
(perhaps somewhat bad quality when you don't know already what it is?)
A simplified circuit diagram is shown. Decoupling capacitors are not
shown. Wonder what sort of softening current is needed for some
improvement? Suppose 13.5V is a practical limit to avoid too much
current transient.

73
Jan-Martin
LA8AK
---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm