2001 Pontiac Aztek slow death

Computer problem
2001 Pontiac Aztek Front Wheel Drive Automatic

the car will start one day with nothing on the dashboard lit up and working except the fan/ac/heat. The lights will work manually. Witin 24 hours the car is dead, unhooking the battery or jumping will restart, it and she goes for about 3 weeks before repeating. Last night I got in the car had no lights I turn the key first the service engind lights came on, then the fan with a click and whiring sound from the engine for a few seconds. The dash lights came on then everything died. My mechanic is going nuts as am i.