This as happened twice, one month apart. Get into the truck at lunch time, turn the key and the engine fires up just fine. But then look at the clock and it is at 12:00 (the time should read 12:30) and all of the radio stations are reset. No lights were left on. Why is the electrical shutting down? I know that the trucks are made so that if any lights (interior or exterior) are left on after the engine is shut off and the key is removed that the electrical will be shut down to the whole truck (ie. the reason for the clock and radio reset) But no lights or any electrical device is ever on. At first I thought that it might be the battery(3years old) but the truck starts up just fine even after I realize that the juice was killed. Are the trucks also designed to shut down any and all power pulls from the battery when it senses the battery getting weak in order to conserve a charge???? I have never had a battery cause these symptoms and start a truck up just perfect?
Any comments would be appreciated.

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1997 4.2L F-150 XLT supercab