Upgraded to 6.7.2. Server seemed unstable and was shutting down and then going into reboot loops, but appears to be the CPU water cooler finally giving up rather than anything specific to 6.7.2. Have fitted new CPU cooler and will report back if any issues remain, but so far looking promising.
Asus M5A99FX Pro R2.0 with BIOS 2501. AMD FX-8350 CPU. 16GB RAM.