Ledfyr says:
I also have problems with junk notes and program changes from my arduinoboy (junk note happens once every few minutes and program change more seldom, so this is not a big problem - but still frustrating).
I get these problems on both my Access Virus C and SammichSID synths.
On my Virus C I can work around it by disabling program changes and setting key range (the junk notes are usually very high).
Maybe this is also possible on the Slim Phatty?
Good tip with the Y-command and using a powered MIDI device. I will try with a MIDI merger and see if it works better.