Hi there,
I too have experienced this with the GA-Z68X-UD3P-B3 motherboard with U1e BIOS, running the same CPU as you. It's very frustrating and so far I have not been able to resolve the issue.
Like you, I thought that the XMP profile was causing the problem initially but setting the system to optimized defaults only cured the problem temporarily and even setting the RAM timings manually didn't help, with the system constantly rebooting on start up and with occasional O/C warnings when it finally decides to work, which is very frustrating especially as the system is not O/C'd at all. However, once the system has thrown its hissy fit and BOOTS everything is working fine.
Sadly I haven't been in a position to expend a lot of time on trying to get to the bottom of this issue so I do not have a fix or reason why this is happening and I don't think flashing back to the AWARD BIOS will help as this is one of the main reasons I wanted to switch to the AMI BIOS in the first place.