1996 Pontiac Grand Prix Car doesn't stop but lights on dash

Electrical problem
1996 Pontiac Grand Prix 6 cyl Automatic 78000 miles

Last week my car died and then when I went to start the car all the dashboard lights would flicker and make the sound that the signal makes. The mechanic found a loose wire and fixed it, they determined that the starter, alternator, ignition switch, battery, cables were all okay. The radiator was replaced last year. I drove the car for five days and the same thing happened again. I drive prety local and do not take the car on any long trips. Both times it took like 30 minutes to jump. And the second time it made the same sounds, the lights flickered and the engine would not turn over. The mechanic is unable to find what is wrong.