Audison AP8. 9 mutes itself off for 10 min after 20 min playing an audio

My AP8.9 started to act weird after 4 years of use. After a longer drive and listening in low volume (actually it doesn't matter how high volume of reproduced audio is), AP8.9 starts to mute itself for exactly 10 min then plays 20 min and exactly after the 20 min period elapses it mutes itself off for another 10 min and it goes around again and again....

When I disconnect the main power of the unit it resets itself and plays ok, but after a while it again cycles 20/10/20/10... I upgraded to the latest firmware and also reset to defaults and configured it from scratch, but nothing seems to solve the problem. Any help would be appreciated.
Many thanks