It's JTAG'd. It seems to be behaving better at the moment. Maybe there was a dodgy connections and disassembling the HDD case and putting it back together fixed it. I reflashed it as well although I only had a dump made just after I flashed it with 15574, so if that was corrupt in any way I guess the dump would be too. If I find it still acting up, I'll try flashing it with a newly built 15574.
I was able to play RE6 for several hours yesterday (this was before reflashing it today), whereas before it would just crash with a dirty disc error (running from HDD) after the first cutscene (before the game actually started). I still get a Fatal Intercept crash when trying to load my savegame for LOTR:WITN but it works OK if I start a new game, so perhaps the savegame's just corrupt.