I've had a similar issue on my different BitX rigs from time to time and the answer usually lies with adding a stub, improving grounding near the rig, using an antenna tuner for more isolation, or even adding a simple RF choke by coiling 6-8 turns of coax. The most recent was with a popular EFHW portable antenna that was a 1:1 (or very close) match, but on 40m SSB, would sometimes send the rig into the menu. Adding about 10ft of counterpoise at the antenna fixed that one.
=Vic=
Vic WA4THR