From experience some samples of PIC18F2550 don't work properly with the USB firmware version from http://www.megadrum.info/content/downloads. Try my alternative USB MIDI firmware for PIC18F2550 from this thread: http://www.megadrum.info/forums/viewtopic.php?f=3&t=268 . Note, that in this case pin26 of PIC18F2550 must be connected to the ground over a 22k-47k resistor.
