flex plate issues AGAIN !!

my flex plate cracked a couple of months ago, so i replaced it, and managed to find another flex plate near by, but it was used, but i still put it on, after around 100 miles it cracked again,

but the 2nd hand flex plate i put on, wasnt 100% flat, it was slightly bent, and when i bolted it up, it straigtend it, but came to take it of today and when i took 1 bolt out it just bent back again,

i have thoughts that the owner before me, when he replaced the engine to a 318 he must of wrested it on the flex plate and it weakend it,

but why should it crack twice, could it be that the 2 old flex plates were weak, or is it my box, i drove the car today just to give it a little bit of a run, and it drove spot on didnt miss a beat, apart from the tapping from the flex plate

could it be that it is unbalanced some where,

anybody got any ideas,