I don't know about the random render times, unless you juiced up settings on a light or shader, I can't see how that happens without messing with the render settings.

How much RAM do you have and what OS are you running? I was getting that out of memory issue a while back, I wasn't surprised as Vista is a resource hog, and then running Maya and MR ate up what was left so I just got more RAM and didn't have that problem again. I did however, turn this notebook into a lab rat to test out Windows 7 to see what all the hullabaloo was about and so far its pretty damn good.