Breaking Flex plates
The nose of the convertor rides in the crank, if the crank of convertor is off much this will happen. Also a bad front pump in a trans too can cause this. '
I do not think it is a weak plate, I think you have a wobble issue, fit the convertor in the crank and see if it is snug, offset etc.
You can mic the convertor and check roundness, but only in a lathe etc can you make sure it is centered>)