hmmm this sounds like what happened to me a few years back when I started up my original A500 with 1.2 kick.......my problem was solved when I did a few things as follows:
1- removed A501 512Kb expansion card
2- opened up case and reset kick rom chip
also I would check if your PSU is kicking out the right voltage by checking with your multimeter
hope this works as I know how you feel when your original miggy goes funny
