I had problems with random freezes and it turned out to be related to the power draw. You can try using the same PSU for the GPUs and their associated risers to it. If it does not help, I'd go with checking all the cables and using treanski's advice.
never use fucking sata i already said it...not with adapters not any other way, only 4pin or 6pin molex
since i stopped using sata for powering risers 99% of my problems are gone
Getting rid of random freezes is a real pain, took me two days going step by step to rule out possible problems.