I've had this problem since day 1 of playing the game in CB1, but from what I can gather it's a system problem rather than a game problem.
What happens is your system is reporting (somehow) that you are using more memory than your system has (I have 2 gigs, it's reporting that it's using like 4.. Though when you add it all up it's not even using close to the 2 gigs it has) and the system then clears whatever the oldest memory is.. Unfortunately, that causes the game to close down it seems..
This may or may not be what is actually happening, but it is what SEEMS to be happening from what I can gather. Here's what you can do to try to avoid this problem:
1. Restart your computer.
2. Avoid running other memory-intensive programs while playing MWO (Yes this is a tough one, especially with the bot system..) - Photoshop causes this problem fast for me..