Check the bonding of the engine to the battery ground.
Check the ground plane on the antenna is bonded to the ground point.
Check the routing of the mic and PTT wires from the SPA 400 to the sockets and the PTT switch and make sure it is not routed with the generator wiring.
Do a complete bonding check!
As with all wood and fabric a/c bonding is very important and over the years ground straps and instrument panel, etc get dirty and corroded.
Good luck
L