Did you investigate whether your graphics card suffers from the problem I described...? It could explain the problem (heavy use of the GPU, even for short periods, causes it to heat up much more quickly than anticipated in its design - this might cause something like ACPI's passive cooling AKA throttling to kick in).
Yes,I did - I was told that my GPU can't get hot enough to cause and problems. I don't get it - my 7 year old HP laptop played things just fine (Zd7030, P4 3.0ghz Nvidia 5600) and my new one which is superior can't run something like this...:-(