I'm using a MIDISPORT 4x4 with X3 which now works great. I say now because when I first switched to Windows 7 it was a disaster. The problem was with the drivers and it took them a while to sort those out. If IRRC I literally had to re-install them on every system boot to get them working.
I would suggest that you're now experiencing the same but with Windows 8. Have you checked their website for driver updates? Unfortunately there's not much else I can suggest. I wasn't impressed at all with their apparent lack of "up-to-date" support and it was enough to put more off buying anymore M-audio gear in the future.
It's working well for me now but it was a real pain for a long time.