Okay, NOW it's fixed. I dunno how much use this thread will be to anyone, since it was about the grainy textures issue, which was simply fixed by deleting all the mental ray nodes. The problem with renders I finally tracked down by deleting objects one at a time. pPipe4, it turns out, was the culprit. A polygon pipe that existed in the outliner, but not in the scene. Once I deleted that, everything worked fine. Not only that, the frames that didn't crash before, now render much faster. I guess deleting the Mental ray nodes caused that particular piece of geometry to be screwy some way that never triggered any warning.

So there ya go. Two problems solved. Hopefully someone with the exact same problem/(s) stumbles on this thread