RT-AC58U 5GHz issues

Several times per day my AC-58U refuses to work on the 5GHz band.
Looking at the system log I can see the following repeating 50-100 times:
Jul 14 12:59:02 kernel: Switching to Tx Mode-1 Threshold 0
Jul 14 12:59:10 kernel: Switching to Tx Mode-1 Threshold 280
Jul 14 12:59:14 kernel: Switching to Tx Mode-1 Threshold 0
Jul 14 12:59:19 kernel: Switching to Tx Mode-1 Threshold 280
Jul 14 12:59:28 kernel: Switching to Tx Mode-1 Threshold 0
Jul 14 12:59:34 kernel: Switching to Tx Mode-1 Threshold...

RT-AC58U 5GHz issues