Tom's Hardware Guide were using some sort of software to detect when the CPU started to throttle, but I have no idea what the software is called - I don't even think they told the name of the program
I think it was around 3.4-3.6GHZ that it started to throttle, but I'd have to check the review again.
If Custom PC's machine crashed after 5 minutes at 3.8GHZ then it's possible that the throttle technology didn't do a good job on their machine.