So I got rid of DOCP and I'm just running the RAM at 2666Mhz...
Memtest64 didn't show anything at this speed, I didn't run it at 3600Mhz.
However the system has been rock solid since Monday, so it must be either RAM, Memory Controller, or something on the motherboard that doesn't want to work at 3600Mhz any more...
Either way, good shout on it being Memory related, major thanks to both @Skiiwee29 and @Sjaakie!!