I swapped everything over to a new housing (still strawberry, but new power supply, CRT and analogue board), and no more arcing. I burnt it in for several hours with Graphing Calculator and no glitches.

So I plugged in the Yamaha CX5M to play with it and ... it flickered and died. I'll crack it open this weekend; I bet it blew its own power supply somehow too.

Something in the universe does not want me to have a music room.