Tyros 5 - faulty speaker

I’m running a Tyros 5 which works well for about 30 mins and then the RHS speaker drops out. A slight hum can be heard from the speaker suggesting a keyboard fault rather than an amplifier / speaker fault. Similar with headphones where the RHS phone drops out. Switching the keyboard off and rebooting it brings the RHS speaker/phone back to life.
If the keyboard is on but not being played it will shutdown after somewhere been 15 mins and 90 mins.
Any ideas?