I think several others have seen a similar issue before, both with really big 'all text' files as well as with files that had lots of pix in them. But RTFs seem to be a lot more stable after the Firmware/ConnSoft update. So, I guess I'm asking if you've done the update, and if it still gives you the problem afterward?
I had one file that choked the Reader (went into a spontaneous soft-reset) when I went to the L size text before, but it takes it in stride since the update.