MarkT:
So its possible that 330mA is not enough?
no probably not.
i just found out that when i power the arduino sequencer using usb from my pc, if the computer is charging (attached to an external power supply) the synth starts going crazy, the same way it would if the arduino itself were powered from an external power supply.
simply disconnecting the laptop from it's external source solves the issue.
i seriously have no idea of why it behaves like this