i'm going to make this as clear as i can, starting from the beginning:
i have made a sequencer using an arduino mega, that outputs 0-3 volts.
the sequencer on it's own works fine powered either from the USB or from an external supply.
obviously a sequencer on it's own isn't very useful, and so i connect it to my synth.
here's where the problems begin: when i power the arduino sequencer from USB, the synth and the sequencer both work fine. 
instead when i power the arduino sequencer from an external output, the SYNTH starts behaving in a strange way (leds flickering, parameter values changing...), while instead the arduino sequencer itself looks fine and does what it's supposed to do.
i know for sure that the problem lies in my project because as soon as i disconnect the sequencer from the synth, the synth stops acting weird, and if i connect the sequencer back to the synth, the problems start again.
MarkT:
The USB supply provides 0.5A normally, what is the rating of the 12V supply you are using?
input: AC100-240V 50/60Hz 180mA
output: DC 12V 330mA
i hope this clears things up.
and thanks for helping anyways XD