One speaker in my stereo fades after a few minutes of play?

[imparicial]

After some ten minutes of play, one speaker begins to fade, first its volume goes down and some beats start to sound dirty with a static like noise. Then it just fades completely, if I turn the volume up then down it works again, but for a moment. My dad has the same system, I connected one of his speakers in the place where mine fades, and it faded too. Anyone knows what causes this? It's a sony mini hi-fi MHC-GX45.
Thank you!!!

 

[Sipan]

lalaaaaaaaa