[LAU] digital input sync problems with M-Audio Transit

Hello, I'm running Fedora 8 Linux with the CCRMA realtime kernel and a M-Audio Transit USB audio interface on a laptop. The interface works fine when there is no digital input (sync is set to internal) but when I connect a digital signal to the optical input (sync seems to go automatically to external) the audio output starts to have sporadic glitches and noise bursts (of course, I stopped Jack before plugging the optical cable). I tried the same thing booting in Windows Vista and I didn't have any problems at all. Is there anyone around having the same kind of issues? I'm using the latest version of madfuload ( http://usb-midi-fw.sourceforge.net/ ) Thanks! Hector