Lack of power?
Have you tried different memory simms?
I've had trouble in the past with different BPPC cards behaving differently with the same memory modules, all of this when working in PPC mode. In particular, the 240Mhz models would be very picky about the standard 128MB HP modules out there that everyone uses.
Additionally, plugging a floppy power connector directly into A1200 motherboard floppy pins may provide more stability.
