i have seen this before..
there is a 8pin IC on sound board that is a 'digital pot' that is failing, try freeze spray when playing up and then check component associated with this ic on the input side to confirm (including cable/header).

repaired a giligans with this exact symptom..the manual control seems to work fine still too which is really misleading.. like when it gets an update it can do it but loses it soon afterwards

regards

PS this should be pretty easy to find