When I tried to have a look what the BIOS had changed the timings to after reverting to DDR-800 MHz I was met by this error:
Check your ram timing entry and use the default jedec timing, try cl8 or cl9 for 1866, 990fx don't support 2133 unless you OC it from there. I think the problem is the compatibility of your ram modules.

You have entered an invalid value in ram timing that's why it prompts you for an error. try using the default timing as I've said.

goodluck...

PS

I've emailed ASROCk for the last 7 days and they didn't replied to me yet, I've got problems configuring my ripjaws ddr3 1600 @ cl6 timing, and also requested them to include mem multiplier options in the uefi. Hoping they would reply to me...