Ground Hog, I also have the same issues you're having and I've determined that unfortunately there is no answer to this issue; memory, overheating, drivers, cfg files, this and that  could be an issue, either separately or in conjungtion, but I do not believe so (I'm no computer expert by any means), chances are the issue is with running FSX under Windows 7, this is a new platform, Windows from its inceptcion has always had issues and they still persist and even different ones are piling up over the older ones . This problem is so intermitent that it is almost impossible to diagnose, let us not forget that we are not running FSX as it comes from the disk, we've added tons of addons, such as aircraft, scenerys, textures, missions, etc. etc.
I've read as much as possible about this issue and haven't found the correct fix yet, sadly. Just to give you an idea, sometimes during my flight session, which last for about an hour I don't have any freezes at all, sometimes only one, yesterday I had 3, go figure, I'm usually also running Plan G, they both freeze at the same time, but it sometimes freezes up even when Plan G is not running, also, I have full control of Windows, I can surf the net and do whatever I want, while FSX and PlanG are Not Responding, at first the only way out was to shut them both down through Task Manager, but if left alone, they'll start running in about 6 to 10 minutes as if nothing had happened and I'm able to fly the entire session without any problems, except yesterday, where I had 3 freezes (I was also flying a new airplane, new install), perhaps this had something to do with it.
I'm going to try next to fly stock airplanes in stock scenerys and see what happens, perhaps it is our weather in Tampabay that doesn't help. By the way, my rig is only about 6 months old and runs like a champ, 4 cores at 3.5 GHZ running without any changes, all temps are in the cool zone, I also fly Rise of Flight and Over Flanders Fields with no problems at all, so this issue  has nothing to do with hardrive ready to go out or old drivers or whatever one might think of.
Man, I wish I had an answer, as I really enjoy this sim, more than any other one, if you find the answer to the problem, would you please shoot me a line, will do likewise.
Best wishes,

Nope, it didn't work, Yesterday had two freezes, go figure what makes this thing tick! Grrrrrrrrr.....!