I use a few different usb midi devices, and don't often have connection problems (sounds like yours is losing connection after a while?).
My first thought is how are you powering the axo? And what are you trying to connect now? Could it be using more power?
I have some weird issues sometimes that I think are power related. Like, I use the dc socket for power, launchpad pro connected, patch running fine. If I plug in usb I think my crappy laptop does something funny and the launchpad crashes. Seems to happen when I have lots of leds lit up. A restart sorts it, and it doesn't happen often. Probably totally unrelated to your problem!
Did you spill paint in the axo?