Did it again this evening. Turned pc on it sat on full fans and blank screen for a bit, then rebooted. Rebooted as few times more before finally starting. Went into bios and found defaults loaded again. Loaded saved settings and reboot - came up with asrock screen and startup menu, then rebooted etc.
Powered off pc then switched off power supply. All back on and started up first time with no issues at all, xmp in place etc. Don't know if this is an issues with 2.46/2.50 or my pc, but hasn't done this with 2.10 and earlier. May revert back and see if it behaves itself again.
EDIT - Again this morning, although even switched off power didn't help either. When I finally got into bios, it had even reset the time too. Gone back to 2.10 for now, which has booted up fine. Will see how it goes over next few days.