Not too sure the problem, sounds though like its more to do with the scene over the comp or MR, maybe have clean up.

Also if you've got a lot of single procedural textures there convert them to textures as they hog RAM, there quite afew ways round taming that RAM and speeding up renders, have a google for em

Though saying that aint a clue why its going up and down so much, you sure thres not a process running in the backround? Maybe start the task manager up and have a look?