Radio/tape Cd Player problems of the 1998 Oldsmobile Bravada

Total electrical loss causing engine shutdown. While driving with the ac on and the radio on during the day, within a 3 mile trip we experianced total electrical failure 2 times. The idiot lights will flash, the dash board needles will bounce and jump, and then everything shutsdown. Engine, brakes, power stearing, lights everything. We were able to drift to the side of the road, shift into park and restart the engine without any problem. If this had happend when we made the last left turn, we probably would have been nailed by a semi. The dealer has had the Bravada from 2 days without finding anything. They can not reproduce the problem. The battery is fine, the terminals are fine, they tested the computer and that is fine. I refuse to let anyone drive this until something is found and fixed.