This reprint of an earlier exchange regarding stability of cambers settings might help. I and another GTO autocrosser were seeing our camber settings move. Mine was changing after reusing the stock bolts more than 6 to 8 times.
(From a June posting

I've had some recent success. I'm running Kumho V710s on wheels that have 1/4 " more offset than the stock wheels. With this setup and the camber adjustment screws backed off I'm able to get 2.1 and 2.2 degrees negative camber measured on the Hunter alignment system. This is with the wheel assemblies pushed up against the stops.
The tech manual says to first torque the strut to wheel assembly bolts to 64 ft lbs, then 72 ft lbs, then using a special GM service tool rotate the bolts an additional 90 degrees. The same manual says to replace these bolts each time you adjust the alignment. The set of four bolts and four nuts runs a little more than $40 retail.
I found that getting 90 degrees of additional torque requires a different force depending on the "age" of these bolts. My theory is that the bolts get stressed or work-hardened as they are torqued to the additional 90 degrees and become less elastic. After reusing the bolts the torque required to get 90 degrees progresses from about 100 ft lbs with a standard click torque wrench to 130 ft lbs* without getting more than 45 degrees.
*130 ft lbs worked the last time out and the camber held, but when I went back to reset my stock camber setting I found that I had a stripped nut and had to cut it off. I broke down and purchased another set of bolts and nuts. I found that the new ones worked and held the camber with the 90 degrees extra rotation at the lower torque.
My routine has been to realign the car before and after each autocross event. On a couple of occasions I've tried to push the negative camber limits for the stock tires and wheels of about 0.5 negative degrees and created a rubbing tire. This has caused me to re-adjust and then re-torque. I believe I re-torqued the failed bolt between 10 and 12 times.
My new plan is to re-use the bolts no more than four or five times or until the 90 degree rotation takes more than 110 ft lbs. At his point I assume the bolt clamping through heating, cooling, and the associated variation in loading is no longer predictable.
I'd like to find an equivalent bolt and nut combination at a lower price, but right now it looks like a more regular replacement schedule is the answer with newer bolts providing enough clamping force with less torque. I'm also considering replacing the stock wheel studs with a set long enough for me to run spacers and constant autocross setting. (I'd be okay with the trade off between re-alignment time and added wear on my stock tires).