oK then its not the driveshaft if it did it before. I would check if the flex plate is the right one for the engine first (internal balanced or external) and also if it threw a weight. You sure it is not a suspension issue like a un balanced wheel? I had a weird one years ago a weight came out of the front rotor kind of acted that way. Just ideas
I know it's not the flex plate, car has new tires freshly balanced. If the driveshaft that was in it was too short to begin with...then when I had this one made then I just repeated the problem...that's why I point towards the driveshaft