I'm not sure if I understand your problem correctly, but let me offer you a few notes. First off, when you turn the ignition switch to the Start position, all power to the accessories is supposed to be cut off. This is done to assure that the starter is getting the full attention of the battery.
Other than that, it sounds like a bad ground problem some where. I would double check the negative battery cable, both where it attaches to the battery and the vehicle. I would make sure there is a good ground strap between the body and the engine, and between the engine and the frame.
When this occurs, do you lose all power to everything, or just your amps? Does this prevent the engine from starting? Have you tested your grounds and power leads when this problem occurs?
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