Working on a s80 for a friend, 150k miles, ran perfect when he bought it then started running bad, dead battery, dash lights go crazy, comes out in the am to headlights on, blinkers flashing.

I have replaced the battery, on our 3rd different brand of alternator, checked all the cables and connections and fuses. When you start the car everything is perfect for 10 seconds or so, then the motor revs up to about 1200 rpm, gauges die, motor shutters, and at that point the alternator isn't putting anything out at all, during that first 10 seconds alternator puts out around 13.8v. After the issue arises, turn the car off wait 10-15 minutes and it starts perfect for 10 seconds again, and the problem continues. Please help