Game stuttering

I have my second alienware 17 because of stuttering. It has 2 ati cards quad 4 processor, 6 gigs of memory and 64 bit operating system and I have been through vista to windows 7. After a certain amount of play it stutters audio and fps drops to nothing. On random nights I get several hours of great play. Cod 4 and 5 left for dead. But most of the time after a short time I get a stutter that brings the game to a stop. I have installed and reinstalled all of the drivers and done it with and without alienware support help and have done it multiple times in all orders. I have also been the upgrade the bios route. Support has been great. They have even built me a new computer all upgraded. Same problem. I have sent them the many sites that talk about it being a heat problem and they are looking for the "right" heat sink. I have tried to put a laptop cooler under the machine but I don't think it helped. The only difference I see with windows 7 is that the frequency of the audio break up has changed to a reverb sound instead of high pitched static. I have no clue what that means! It seems to get worse with increase time and increase action but then out of the blue it runs upto the time I must restart. I have had this problem with nothing installed but a logitech z9 mouse and a zmerc keyboard so I reset the system to factory and played without keyboard or mouse - eventually the same problem occurs. This laptop is fully loaded and I am beginning to wonder if that is the probem. Any Ideas?