I have now tried the MaxMem setting and this didn't seem to have any effect. Next I installed the beta 169.04 drivers, this seems to have improved things. I managed to get two games that lasted 20 minutes before a CTD occurred - a record for me. Unfortunatley the CTD was terminal and I had to reset my machine.
The irony of all this is that with the GTX, this game runs very smoothly on max settings and plays extremely well - when it works. I'd like to buy their addon packs, but I will not do so until Bohemia or nVidia sort this out.
RobP