my serpentine keep braking, why?

would hazard to guess that your tensioner is not right...it's over- doing it, during critical times and causing it to break...take a look at your timing belt tensioner...should be on a spring and allow some travel...not cinched down to beat the band~...perhaps the replacement personnel did not fully understand this belt tensioner...and how it works exactly...NOT tight...but some travel~...ok, check the tensioner to make sure there is some 'give' in the installation this time~

Check and turn by hand all the accessories the belt goes around making sure none has seized up. Also check the pulleys on the accessories to make sure nothing on them that would cut belt when running.

Firebird has the best suggestion. Serpentine belts are pretty rugged so there must be an issue with something that drives the belt.