Hello,   It's possible that the first three symptoms you listed are all part of the same issue. If you haven't I'd try reseating your risers, cables, and PCI devices. If one of these persist, there may be a more serious issue requiring hardware replacement.    For DIMM A3, you most likely just need to replace the DIMM. Still, if you want to be sure, you can test it in another memory slot to see if the issue follows the DIMM.
Got an R720 that's producing some errors in the log. The first is a "bus fatal error bus 8 device 0 function 1." The second is System Board Riser 3 cable or interconnect is not connected or is improperly connected. The third is CPU1 machine check error. Finally I've got multibit memory errors on DIMM A3. I'm going to switch the A and B RAM and CPU1 and 2 and see what happens, error wise. I don't know what's at bus 8, though, but that error makes me wonder if the motherboard has an issue. Could use suggestions and advice. Thanks!
R720 Bus Fatal Error