Pentium x3450 not working in ga-h55m-ud2h

[pedant]

So I decided to upgrade the CPU in one of my computers.
However, the computer just goes in to a few second reboot cycle with no beeps.
I unplugged most accessories but same behaviour. Tried reseating a few times but same issue. Clear CMOS, same issue.
The original g6950 still works in it.
I then tried the x3450 in an old HP 110 and it works fine.

The gigabyte BIOS is latest version which should support it.
http://www.gigabyte.com/support-downloads/cpu-support-popup.aspx?pid=3309
The PSU is a reputable 500w which should be fine.

So I'm stuck. Any suggestions on things to try? For the moment I've put the x3450 in the HP 110 but would like it in the gigabyte.

Thanks.