Issue using Telem 1 and Telem 2 simultaneously on Cube black

I’ve been fighting with a problem with telemetry modem on Telem 1 while using a Gremsy S1V3 gimbal on Telem2. The modem and downlink works fine, link quality around 90-100%, if the gimbal is unplugged, but when the gimbal is connected, the link quality is below 50%, often below 35%. This is absolutely consistent. This causes problems reading back the mission and in flight the telemetry often drops out entirely.

I’ve tried an OSD on Telem1 also, and that doesn’t update properly when the gimbal is on Telem2 either.

I’ve tried powering the modem from the serial port or via the BEC, it makes no difference.

The gimbal is attached using the cable supplied with it, which has ground, Tx and Rx wires only. It is powered by a separate BEC.

I think I tried connecting the Gimbal to GPS2 and setting up serial4 for it, but never managed to get that to work. I think I’ve been unable to get an OSD to work on that either, are there reaitrcitions on what that serial port can be used for?

The gimbal works fine.

I’ve tried swapping another Cube black and the issue is the same with the gimbal on Telem2. I also had 4.0.5 on the cube originally, and tried upgrading to 4.0.7, which didn’t fix it. The other cube was on 4.0.7.

Any ideas?