Seems unlikely to be a power issue if it does it with just the A1200 and hard drive with no accelerator connected. Try disconnecting everything non-essential (including the hard drive) then boot to a Workbench floppy, and see if that hangs. If it does, then it may be that you have a problem with the Amiga itself. Perhaps Amigakit's suggestion of checking the positioning of the motherboard is the key? Also make sure none of the metal shielding is shorting any parts of the board.
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moto