Lately when I start my car, there is often no power to the headlights, power windows, seatbelt reminder light, power sunroof and turning signals. Also, the transmission will not engage unless I use the shift-lock override button. At the same time, however, the parking lights, power mirrors, radio, and hazard flashers all work. When this occurs, if I push the shift-lock override button, engage the transmission and start driving, then within 15-20 minutes everything starts to work again (I am alerted to this by the seatbelt warning light flashing on temporarily and then going out). Once things start to work, they don't stop working again while I am driving. If I turn the car off, the problem may or may not repeat itself the next time I start the car, but sooner or later (usually sooner) it happens again. Can anybody tell me what is causing this?