i have a similiar problem : after some time working in logic ,approx 2 hours, my usb midi interface stops working , i have to unplug - replug and it works again .
this happens since 3.0.3

it looks like not only Mac users have this problem...
I run Vista and my virus is recognized properly by the system. midi, audio drivers etc. installed all properly it says in the system settings.

however, when i try to install the 3.0.3 drivers again, first the virus is recognised (asking to plug out and plug in the usb cable), then one step later the installation does not find the virus anymore :(

seems like a bug in the driver or something... so I get no midi signals in either direction into any app as well.

:)
ok resetting the virus solved my problem:

1. Disconnect the USB cable from the TI
2. Disconnect the power cable from the TI
3. Reconnect the power cable to the power outlet
4. Reconnect the USB cable to the TI

____
actually, I did not need to go into the next step, but just in case:

5. Press and hold the ARP EDIT button for a few seconds (10?) and then let go