Mysterious Problem From Hell!

Here's the problem, would really appreciate any good ideas...

I have a game, Empire Earth - Art of Conquest. The graphics and sounds are very good. When I first got this game 6 months ago, it was great. A few weeks ago, the graphics got very jumpy, like the CPU was struggling to keep up. I don't know why, but these are the things I have tried:

1. Checked for worms, trojans, viruses, etc, in case the CPU was having to do extra work. Nothing, I am 100% sure of that.
2. Changed the game settings to low quality / high performance / different screen resolutions. Nothing on any combination.
3. Read the game readme file (amazing!). There are some potential problems with some video cards on certain settings, but they don't apply to me.
4. Went to Task Manager and set the process priority higher, lower, all the settings.
5. Stopped all background processes, including TSR programs, anti-virus, firewall.
6. Reinstalled DirectX.
7. Reinstalled the video card drivers.
8. Reinstalled the game (actually reinstalled almost all applications just in case).
9. Increased the page file size.
10. Decreased Page File size.
11. Changed the sample rate and buffer sizes on my mouse.
12. Used my old mouse (not wireless optical, but wired and analog).
13. Uninstalled mouse drivers (as Logitech drivers can cause problems sometimes).
14. Registry cleanup - found a few problems but not the one that I need. Fixed those anyway.
15. Tried older drivers for sound card, video card.
16. Optimised just about everything I could think of.
17. Used a tool to free up memory.
18. Tried every combination of all the above.
19. Got pissed off and clean installed both Win2000 and WinXP. Tried it with and without patches from our friends at Windows Update.

The CPU is not doing any extra work apart from any applications I run. But for some reason when I run applications that use heavy graphics (or maybe sound) it runs out of memory fast. Pausing applications lets the CPU catch up again, but soon it falls behind again. I suspected a hardware problem, but a week of testing has failed to turn up anything wrong.

It seems to be not only the game I mentioned, but possibly also the DVD software as well, as well as other video/sound-intensive applications and games. Anyway, Windows is still running fine and all my applications that don't make extensive use of graphics are fine - 100%. I'm sure it's not an O/S problem, but something simple yet obscure.

I believe it is possible that the problem only affects Dell Inspiron's (nothing would surprise me any more with bloody Dell computers).

The strange thing is that it used to run perfectly, even on the lowest performance settings. It is very strange indeed. If you have any brilliant ideas, please let me know.

Have you tried swapping out hardware at all?If you are pretty sure it isn't OS related, that's the next step.