I hate to say it, but I think it's a time to buy a new computer, with more RAM and more space. If you have at least 3 EPs, you need 2 Giga RAM, DirectX last version, and more powerful "engine".
If you already have it - the only one culprit I can think of is - Securom...for some computers, it's "Jack the Ripper"...

. Luckily, my computer stands on the "right side of the needle", without much problems, but only restarting before game.