a few things could be happening , you have a misalignment issue causing stress on the flexplate, a convertor balance issue , or just had a failed flexplate , i had a problem years ago where a car i had would do the same thing about every 6 mos i finally pulled the engine out and found out that the rear flange that the trans bolts to was .090 off from rigth to left causing things to bind and the plate up and eventually crack