electrical problem

I have a 92 crown vic lx. This problem started about 7 months ago. The car starts fine and runs great. This problem only happens about once a month. almost everything electrical on the inside turns off and will not work. The radio, clock, power windows, mirrors, and windshield wipers will not work. it also will not pump washer fluid. The power seats work but run a little slow. The signal lights and hazard lights run really slow( about one blink every 2-3 seconds). headlights work but are dim.

I have checked the fuses and i found out that one of them was burned out. After testing all of the switches i found out that it was the headlight switch. I have not replaced it yet because it still works. It only burns the fuse if i turn the delay off.

With that said, the second time i had this problem i checked the fuses again and they were all good. the fuse for the headlight switch had not burned.

the woltage meter normally reads at about the R. When i am having this problem it reads a little below the O. If i drive the car for about 15 minutes everything turns back on. I took my car to a local mechanic that does all of the major work on my car. He tested the alternator and battery and it was charging.