do you have any turbo attached to a1200? if yes, remove it and retry.
also reseat kickstart rom chips. retry.
clean out the expansion slot. retry.
check your peripherials. retry.
check the power supply. retry. i remember once i lost the -12v line. the amiga booted fine, but liked to hang with "no reason", and the sound was noisy. i recovered the cable and all went back to normal.
check other things such as floppy ribbon, floppy itself...
i don`t think it`s a software problem, unless you have a virus, but this seems to be another story...
