2000 Chevy blazer. 107k miles. Starts up and voltage reads 14. As soon as I drive voltage drops to about 11-12 I stop at a light and as soon as I drive again voltage goes back up to 14 again briefly and than drops back down to 11-12. If I drive for too long battery slowly dies and car shuts off. New battery, new alternator, 2 mechanics have looked at it and can't figure it out. Someone please help!

 

If the serpentine belt and the serpentine belt tensioner haven't been replaced, start there. Both of those parts wear out, have maintenance inspection/replacement intervals (which are rarely followed) and both can cause your observed problem.

Do that before messing with the alternator or battery ... errr, never mind.

 

Good advice, and I'd also be checking ground wires. Battery ground is likely clean at this point but check where is mounts to engine and chassis as well.