Reply to dj-tech i-mix stops working on vdj

Hi,
I bought dj-tech i-mix midi controller. I have a huge problem. When I run it on my virtual dj it works for 30 seconds or something and then it just stops. Led lights on the controller are still working and virtual dj is still playing but when I press something on my controller, nothing happens.
Then I plug usb off and then on again and it starts to work. I tried on different computers and different operating systems (win xp and windows 7) but its still not working.
Any help ? thanks in advanced.