Raja - Sorry its taken me a few days to respond but I have been carrying out further testing to resolve the issue.
Note that the issue has been resolved...! but the process of elimination and resolution was somewhat lengthy - however I'll try and keep my reply brief.
The next few points will seem to have nothing to do with the UEFI locking up with profile selection but my memory failing when running Memtest led me to resolve the issue - So I am just posting this up for any others who may come across the same issue.
As mentioned in my opening post - I was having issues with the UEFI locking up when loading profiles. Another issue I had with this new build was that my memory would fail Memtest only 30 secs or so into the tests. Note that testing within Windows was 100% OK - It would simply fail when testing with Memtest in a DOS environment.
At first I had assumed that these two symptoms were unrelated - however it transpires that I was incorrect.
I had removed the memory and returned to my local store where they tested the memory overnight without issue. I then loaned some different memory to try in my setup which also proceeded to fail in the same way. At this point I had thought that the MB was faulty - Again wrong...!
My system runs a custom water-cooled loop which makes it a bit of a pain to remove and install the MB. However I had to remove the MB and I then proceeded to test on my bench with the Intel stock cooler. At this point I noticed that the memory was testing OK. So this confused me at this point in time.
To try an isolate further - I also swapped power supplies to no avail. I got to the point where I was running out of options. The only thing left was that when the MB was placed in the case, the only extra bit of hardware connected to the MB was my Aquaero 6XT controller. This connects to the MB via a simple USB header. When I was testing on the bench with the stock Intel fan cooler the Aquaero was not required and therefore disconnected from the MB.
To cut a long story short.... The Aquaero computer was plugged into USB port #14 - This is the one also utilised by the ROG Extension panel.
For whatever reason when the Aquaero was connected to this port - it would cause Memtest to generate false positives as well as causing the UEFI to lockup when recalling saved profiles...weird. In a Windows environment everything seemed to be A-OK.
Anyway - moving the Aquaero over to one of the other headers resolved both issues.
Obviously the shared ROG-EXT USB port is not quite the same as the other internal USB ports....!
Only wasted a few days on this ...:D
There may also be an error in the manual as on pg1-41 it refers to USB #13 on the ROG_EXT header but I found that USB #14 is the one that was working for me and the lower 4 pin port was not.