Mustang II Control Arm Slip

[zimspad]

I am using a Mustang II setup on my 51 AD and have had the upper control arm slip 3 times in 5k miles. I know what your first thought is, I didn't have it properly tightened 3 times. While that was probably the cast the first time, fairly confident it wasn't the second time, and I personally supervised the torquing of all 4 nuts to factory specs the third time. It is a Heidts unit. I am using factory hardware, not after market T-bolts. The spring perches have no damage or warping that could cause them to slip. I have done some searching and found that others have had this problem, one even references a "retainer" that they made, but have found no specifics on how to remedy the problem. Anyone dealt with this and found a solution that works?