Audio 8 power issue

In our DJ group we have NI's 2x A8dj, 2x A4DJ and soon on the way A6DJ.

The A8s are oldest, and from time they entered the market. One of them started to have problems. When connected to windows (none of us has mac) it didn't appear. Instead windows reported that USB device is not properly connected or something about it doesn't run on efficient speed. The green USB symbol on Audio 8 was blinking, signaling something is not right. Anyway, the problem was fixed when you rattled a little bit wit usb cable on soundcard's end. Then you tryed to connect soundcard back and everything was fine. The problem was getting worse over time (a month), always it needed more careful approach to find in which position cable worked (of course we used loads cables, and i have some high quality DJTT and even some low quality usb 2.0 cables). obviously it started to droping out during playback. And after a week we couldn't found any position of cable to work at all. Blink green usb sign was all i got from it. We have sended the card to official service of course. Problem is that i don't have any ideas what's wrong and how much they are going to charge us. At first i thought it's problem with physical port itself (like when cable moves, it looses powev connection and it drops)... but the more i think about it, more suspicious i get.

now i have noticed that other A8 had same problem week ago (just once or twice), and i'm suspicious that i have seen this issue on one of the A4 before as well.

Anyone any thoughts?

btw it's not laptop fail, as the problem was on every machine.