Are you using a laptop? Some USB ports don't push out enough power to keep the midi device operating correctly for long periods of time. I had this problem on my laptop. There is only one port out of 4 that works all the time. http://www.ableton.com/forum/viewtopic.php?t=48997 That thread should poi...

Cheap for MAC: For a sequencer/host: http://www.renoise.com/ $65 usable demo available and a good tutorial page to help understand whats going on. Free VST & AU: http://www.dontcrack.com/freeware/software.php/id/7030/audio/Virtual-Instruments/platform/mac/ Lots a good ones there, Automat & C...