I am not by any means disputing your results as that would be futile, however if I may point out my experience with S3's, it is very rare that you find any two units hashing the same, even when using similar freq & voltage settings, PSU and connected to the same pool.

In your case however, that you have experienced the same wall of performance after the first hour, and that happening on most of your units, leads me to believe the issue must be local to your setup.

My first instinct was a heat / temperature problem, however, you seem to discount this as a possibility, and that the units run as normal on stock frequencies seems to justify that. I am not completely sold though since I have never run an S3 variant at temps as high as yours. Before the season change and my moving my rigs outside, I had them all running the fans at full pelt, aka blue wire hack, which kept the temps low, and now that the seasons have changed and my rigs reside in the garden, my temps are a lot lower.

My second guess is how you are powering your rigs. Though you appear to have the right PSU's, this may be a cause if you have them all plugged into the same extension lead. They may not trip your circuit, but they will compete for power, especially if you are running them off a 110v circuit.

If you choose to attempt the OC again, I wish you luck, but I think in your instance I have given as much help as I possibly can, bearing in mind I am neither an engineer nor an electrician.