I need help with electrical question. Have a6 quattro 2.8 1997. Has been running perfectly. Was driving home from work and when stopped at sign car stopped. When tried to start completely dead - turn key but nothing happens. Called AAA got jump and started right up. Ran it for 15 min to charge and made the ten mile trip with no problem. Mechanic said alternator bad causing battery to die. Replaced alternator and car started right up but then stopped after 10 min running. Could not restart so replaced battery. Started right up and ran ten min again before dying again. Was completely dead when try to restart. All power gone again. Asking for help with what would cause the power failures. How do I fix it.