Unstable volume on Logitech 5.1 X-540 speakers

Hello, i have this problem with my speakers which i can't figure out and i hope that i came on the right place for help.
I own this 5.1 computer speaker system by Logitech (X-540), which i got from my friend. The problem is that the volume is very unstable. At some time it's on normal level that suddenly the volume drops for some time and after some time it goes back to normal. It happens that a lot and it's very annoying, especially at night so i have to keep adjusting it.
Fried, from who i bought them, was barely using them, he was mostly using headphones so he didn't notice that problem. They are probably like 3 years old or more i can't tell for sure.

I have no idea what could be wrong, i looked throughout the web but i couldn't find the solution. I hope maybe you could help me, if you can i would be very grateful.

 

No the volume control works normally, I think it has to do something with electronics, but i don't know enough to about it to know. I'll try to connect them to separate socket but i doubt that it will make any difference.

EDIT: I've plugged the speakers to a separate socket, firstly it looked like problem solved but after a while it got bad again, or actually worse. Any advice?