I'm running into the same thing when I have a lot of images open. The RAM usage just keeps climbing and climbing until it shuts down or even shuts the whole system down with a BSOD and the error reads "Memory_Management".
I did some research and found this... "If you don’t render the standard-sized or 1:1 previews then Lightroom is constantly trying to build them as you edit your images resulting in very slow performance."
Once you have all your images open in LR, select all the images and then go to Library, Previews, (select) Render 1:1 Previews. This will take some time depending on the number of images you have, but it will improve the workflow as you go through the images. From what I've gathered, if you don't do this then LR will constantly be attempting to build them over and over and I have noticed by PF usage go up until it crashes. However, when I perform this task I can go from image to image to image and the PF Usage stays much lower and no more crashes. I also have LR2.5 so hopefully it works for you.