Breaking Flex plates
All true and good points without checking to verify 1st, but these are common causes as Moper and others posted.
So throughing a better part yes may work, but that is a cast on the arm for a broken leg.
I do agree it needs to be checked 1st, but I have done 1000s of transmissions in 35 years, and seen many tilted, offset and out of alignment trans to blocks which can cause enough offset to make this failure.
In the old days in a Mopar Bible they even made folks cut every other loop in the plate, and when setup correctly the bigger 8 bolt units rarely broke.
it is all about balance and less flex, no pun intended. :)