Actually, it is a pain why it was set up this way, but an easy fix. I have found that going in to your preferences–>Memory and Cache and set them to the following:
Max Memory Usage: Leave at what it is (mine is at 120%, or equal to 3GB
Image Cache: 33% –This is where the problem most likely occurs, so the lower, the better
Check the Enable Disk Cache, and specify the settings where you would like it to save to
From what I have learned, you do NOT want it to render image cache to your memory. There is not really any need for it. Have it render to the disk. It works so much better
Hope this helps
Tim
Tim