I just had the same problem with a A3000 I just finished restoring and it was the darn daughterboard which just wasn't inserted right.
I pulled it out and cleaned all the contacts and re-seated it and everything then booted up fine.
Not sure if this is your problem but it might be worth a try. I also did a Google search for Amiga troubleshooting and found quite a bit of info, but you have probably already tried that.
Good Luck,
Tim
