Moinmoin,
had the same problem on a stage with "bad electrics" and with lots of connections at the back of the P'08 made. I suspect, that some of the connection earths/masses had some potential-differences they should not have, as I could finish the gig with only power supply and output connected. I lost all MIDI and CV-ins for the rest of my performance, as I did not have the time and coolness to test everything during the gig.
Just teared out everything not
really needed. To my luck and pleasure this never occured again...
So here are my questions:
Does this happen even if
all connections (power supply has to stay, of course) at the P'08 backside are removed?
If not: Which connections to what equipment raise problems and is that equipment OK?
If so: Did You try another power supply (12-15VDC, 400mA or more, +inside, -outside)? You may find one of those as part of Your or one of Your bandmate's equipment, indeed they are quite common.
If the knowledge gathered by answering these questions does not lead to improvement, I suppose to call/e-mail DSI support and tell them Your problem (and what You already did to further investigate)
HTH
Martin