Could you please help with two problems with my amplifier Marshall Code 25?

The first and more serious one is when I turn it on, the sound is ok, but 15-20 minutes later sound dissapears. To make it sound again I need to turn it off and on again or plug and unplug my headphones cable, then sound works again. 

Another problem is with the screen. The screen lost its brightness. When I turn it on, the screen is bright, letters are seen clearly. Several seconds later it loses its brightness. It is still orange but the letters are far less bright.

What might it be with my Marshall Code 25?