I've got one problem with My 2x2

I'm running on MacBook air 10.6.7.
The 2X2 is plugg on a external powered Hub USB (with a enttec USB DMX pro).
All works fine few hours, but after ( 2 or 3 or 5 Hours) it seems to stop working.
There is no more midi IN message (in Midi monitor for example but in Ableton, Steinberg or Ptools too), but the led on the 2X2 continue flashing good, and it still there in "Audio/Midi setup" in mac osX, but no more message incomming in midi monitor.
For resolve it, I need Unplugg the usb of 2X2 (it desepear in MACOS X of course) and replug it , and so it going on for few hours again.
I'm working for a long exhibition, and I need to find an issue.

Please give me genious idea ;-)
Thanx for all