I just tried some Dirt 4 with gpu-z and real temp off, I also turned gpu scaling in wattman off and the stutter's no longer there, I then moved to Dirt Rally and halfway through the first stage I got booted out to Windows. I'm using a TX850 psu so I suppose I'd better grab a 1000 watter to rule that out, It's gonna mean I've spent somewhere from 900 to a grand though, that suck's
EDIT: I just looked and a 1000 watter costs around £200 sod that. If I can't figure it out I'll have to get a replacement.