3XS Evolve NLE 4K - suspected RAM failure

I bought the above system in Jan 2015. Just to refresh memories, it is based on an Intel i7 5820K, Asus X99-S, and 32GB of DDR4 VengLPX 2666mhz.

In the last couple of weeks this system has started booting to the American Megatrends splash screen and reporting only 24GB of RAM.
I've taken out and re-seated the RAM (taking care to replace each card into its previous place) just in case the contacts have deteriorated over time, but no change.

Now, I emphasise that I am not a hardware expert, but I am guessing that either I have some RAM that has failed, or I have a motherboard with a fault. My hunch is the former. At this point I am stuck and don't know how to proceed.

The system is still usable - I make an adjustment on the F1-invoked screen, change EZ system tuning to "Asus optimal"" - Save and Exit F10, and the system boots up into Windows.

Any suggestions appreciated.

Graham Laws