I'm having a similar issue with our G Computer -- The garbled screen, no disc access. We actually have two computers-- just received one from a guy in Germany. It was supposed to be totally working, but it didn't even come with the original floppy drives.
Our main mix computer has these aftermarket cards by the Desk Doctor. They're for G+ SCSI floppies, presumably to mate with a G computer. I'm kind of confused by these two computers which are externally ostensibly the same. Their cards are quite different. The BUS driver in the computer we just received doesn't have pushbutton switches like the other one.
Anyway-- no disc access occurs. The drives power up fine, but no action. Screen hangs on the garbled mess for a second, then clears and goes blank. Any tips would be appreciated.