Can't really think of anything, apart from what chris_s already said about noting if it is associated with any particular codec or track.
Maybe you could try using an _older_ version of RB? Just in case it's some specific commit that has caused a problem?
Also, have you definitely used the original firmware for an extended period of time? My suspicion would be a hardware issue (different recent dev versions of RB are working fine for me on multiple ipods) but hard to think why it would only show up in RB.
Sounds like a very tedious and tricky problem to pin down, if it requires using the thing for a long-ish period each time to get the fault to occur.
When you say the screen goes 'blank', do you mean it whites-out, or the backlight turns off?