Rear motor mount feels too loose!

I was curious as to if any of you have had this problem before. I own a 98 GST and for a couple of years now the motor has just felt like it is not solid in the vehicle. Every time I give it gas it feels as if the motor bounces front to back in the engine bay. I've replaced the motor mounts twice, first with all urethane mounts (all 4) and just recently with all brand new stock mounts (all 4). Its def the rear mount that is causeing this though. I was wondering if you have heard of any fixes for this, otherwise I'm just gonna try to weld some extra support on the rear mount so it holds tighter onto the transmission.