Just an update on this topic, I tried the solution suggested by redxps360, which unfortunately did not work. I decided to try something else that was in the back of mind and that was simply to add the extra 8GB RAM to be the exact same specifications as that in the machine which was working at the time (2x4GB). Therefore I purchased two more 4GB cards of Kingston model KVR1333D3N9H/4G. I have previously tried two of those and two model Kingston KP382H-HYC.  Problem solved.
I have searched this community but unable to find the answer, so any help would be appreciated. I have just upgraded my Son's optiplex 790 desktop CPU to an i7 from i5 and he previously had 4x2GB ram so I bought 4x4GB of Kingston RAM to upgrade to 16GB. The problem I have is that it will only accept two of the modules. I have checked them all, and all are working but will not allow me to add the other two, with the front of the PC showing 1 and 3 error code. I even attempted adding two of the 2GB modules but an error code occurred with that too. The system is 64 bit so unable to work this one out. Any advice, apart from new PC? 
RAM upgrade (Optiplex 790)