How long have you been using those RM750s? I've heard they're not as good as the rest of the Corsair OEM models using Seasonic or CWT stuff.
We have 5 rigs like this running 24/7 for 6 weeks, and the RM750 never failed so far (in fact nothing failed at all).
I had problems only with PSU Lepa 1600W, which where failing all the time because of low 5V amperage capabilities (for risers).
Eric
Interesting the LEPA 1600 gave you problems. They're sold out everywhere for people making 5x280x rigs.