I have two Foxs - one with custom firmware and one with standard. Both up to date 1.03.12. Both on padding.
Last week I set both to record the same program on BBC4 at 7:30pm - one of the "Proms" programs.
Both recordings were scrambled. As it is known WHAT goes wrong and how to fix it, this doesn't actually help. But I don't think there's any knowledge on WHY recording goes wrong. This does prove (or at least strongly suggest) that it is not random, and that it is reproducible. ie all Fox's will fail on a given program.
I did this experiment because a few weeks ago I tried to record a couple of documentary programs about China at around 2am on one machine. they were a week apart, different parts of a series (recorded singly), and they both failed. The common factor here may be the creators or handlers of the program series, ie what they are doing (or NOT doing).