Sounds like a PSU problem.. adding the blizzard might just put it over the edge.
I know it sounds obvious, but have you stripped it down to the bare 1200+busboard ?
Since I`ve never had a powerflyer, this next bit is just a guess.
AFAIK, it plugs into the 1200 via the ROM sockets and then has the ROMs plugged into the powerflyer, and so the ROMs are higher than normal.
If the busboard covers the powerflyer on the motherboard, there`s every chance that the ROMs could be shorting on the underside of the busboard, and giving you the red "bad rom" screen.
So try it without the flyer.. if it boots, then you need to add some insulation .