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