This is a follow up-message for anyone who has a similar problem and who reads this. I have discovered a solution to the problem but it is unlikely to be one that you would stumble upon by chance.
I have 4 memory modules, two of which came with the computer originally and the other two were added later. They come from different manufacturers. I had the 2 x 2 Gb newer modules installed as DIMM 0 Channels A and B and the 2 x 1 Gb older modules installed as DIMM 1 Channels A and B. I reversed the order so that the older modules were installed as DIMM 0 and the problem disappeared.
Why it should be that some brands of module are detected differently to others is very odd.
I hope this might help someone.