Craft destroying decouplers after certain time in flight

[4d4Garrison]

Today I built a craft based on the Saturn V rocket. After roughly 4:24-4:28 into every flight the rocket will destroy the decoupler between the engine mount and first stage, and or the decoupler between the engine on the LEM and the upper stage. I tried restarting the game, and modifying the craft to fix this. I also tried staging away the first stage early, and even the second stage right after first stage separation. Every time something breaks at this time point in the flight. 

This time corresponds with the time it takes the first stage to run out of fuel. or close to it. 

I also tried firing the first stage held down by launch clamps and experienced no issues at burnout. 

I also have a structural failure if I try staging after burning the first stage dry held down on the ground. 

The weirdest part is that I lose control even if the command module is intact and presumably the kerbals inside are alive. 

Now at this point I am confused but begin to notice that perhaps it is an altitude problem. I try again without doing any gravity turns and notice that indeed, some sort of physics gets applied to the rocket when it reaches ~21.5km, however, this time flying straight up nothing experiences a structural failure.  

interestingly even turning on unbreakable joints on I still experience parts breaking when in a gravity turn and reaching 21.5km 

I tried a further test and yet again experienced a sudden buffet of physics forces upon reaching 21.5km. I kept the rocket going without staging and at 4:37 the same structural parts failed and the rocket lost the ability to be controlled. 

As a note for the devs I DO have the stiffness of joints turned up from the standard 1500 to 15e6. 

I don't have a good way to insert or upload images and I am not about to get an account on some website to do so,  but I did take screen captures of a few of the failures.