Static problem Micro-Cube bass rx

Hi everyone,
I have owned a Micro cube bass rx for about 10 years. Given great service for “quiet”practices, jamming in the garden etc.
The last few weeks it has developed a static/electric nibble when I touch the strings. No hum just a click when you touch the strings
Same on batteries or mains and only after the amp is on for 15mins or so. Also same with different leads and basses
Anyone heard of this?
There is a ground point on the back but Roland say where not to connect it rather than how to.also thought about buying a cheap wireless transmitter.

Also also good to buy new toys but seems the functionality is quite unique, don’t want to buy the same thing

appreciate suggestions!

 

Same with different leads and different basses. Only constant is the amp.

 

Thanks