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. Anyone knows what causes this? It's a sony mini hi-fi MHC-GX45.
Thank you!!!
This is not a car stereo, it's a home stereo. Two speakers and a sub.
My dad has the same system, I connected one of his speakers in the place where mine fades, and it faded too.

 

[Zink71]

I think I had the same problem w/ a Yamaha receiver. Basically, the receiver needs some parts replaced on the back where the speaker output is. Some kind of part that may be hard to find. I had this fixed once under warranty and they told me what they fixed. Then it happened again years later when the receiver was way out of warranty. So, I threw the thing away.