Gotta admit, it was the goofiest grounding issue I’ve run across. Basically the output jack tip was coming in contact with the grounding tape on the underside of the control cavity cover (never seen that before) and the tip contact surface wasn’t making good contact with some cables, but ok with others (seen that plenty of times).
Reset the tip contact to the correct position and removed a patch of the control cavity grounding tape so if the top hits the inside if the cover it’ll just hit plastic and not a conductive surface. Ideally I would have liked to have taken a forstner bit to the output jack seat to lower the jack in the cavity a bit but time didn’t really permit. It’s perfectly playable as is but next time Marshall comes over he can bring the bass with him and we’ll fix it up nicely.
BTW - not sure what strings are on it, but it’s a sweet sounding bass if your sound goals are on the darker side. I now have that model Tbird on my GAS short list.