Breaking Flex plates

I agree MOPER & Stroker.....especially about the factory plate takin serious abuse without so much as a wrinkle..... I have also bolted 727 small block bells to just about everything small block without so much as a mouse fart..... I have swapped torq Conv from trucks, cabs, and even from after market (TCI) same game.....I believe there is a goodly amount of room for "slop" and exceeding that is tearing up your plates, which tells me, your issue may be more extensive than you think. Like Stroker says get it checked...... we are miles away from what is actually wrong, because most of us are MILES AWAY!!! Hands on & eyes on is what gets the facts.
Back in the day I have done 4000 neutral blasts on stock 727's hangin off 440's 383's 340's 318's, 6 packs, dual quads......NEVER BLEW a 727 or a flex! Those things are tough! Twisted an output shaft all to hell in a 904 hangin off a stock 340 and the plate was intact....the shaft looked like an unevenly unraveled cable, but the plate was true!
Time to visit the tranny guy or maybe the engine guy or maybe both