It's not a common thing, sometimes the outside bolts are bottoming out so I would pull one out to check the end of the bolt to see if it's bottoming out. It only does it under full load because thats when the SC has it's greatest load, it pulls the SC down and tries to pull that intake off, so if the bolt is not completly tight, it will allow it too leak. Only other option is to brace the front plate to the block to stop the flex.
It also needs RTV silicone.
Thanks,
Dustin