The amplifier its working normally at any volume and the status could be Stereo, Mono or Bridge. It make mute, turn on only both protect leds and mute for a few seconds and keep working normally. But sometimes this procedure repeats many times, I mean the amplifier its working fine and mute without any reason and works fine again, but sometimes too the amplifier works fine all time.
 
Anybody knows what could be happening?

If you haven't already, you may want to give Mackie technical support a call.

If it's out of warranty, I'd call S&X Pro Audio at 206-575-1704.  These guys used to be Mackie in-house techs--Mackie is referring all non-warranty service to them.