Yeah, sorry for the confusion. I originally flashed 15574 (built with 360 Hackbox) with NANDFlasher 360 after which it wouldn't boot. I normally use Flash 360 and have never had any problems with it before (although it's possible that whatever I was using then to build the image remapped the bad blocks, rather than Flash 360) but after it stopped working and I had to fix it with my USB SPI flasher, this time I flashed 15574 (built with Xebuild GUI) from Xellous. So it could have been a problem with NANDFlasher 360, 360 Hackbox or something wrong I did.
Yeah, it's a JTAG and it's been fine until fairly recently, so if the soldering is dodgy it's become so by itself recently. I don't think it's ever frozen during a game (only rebooted a couple of times and that only in the last week) but for maybe the last six months to a year it's occasionally booted fine but with two red lights stuck on, which gets fixed by a reboot. I can't recall exactly when this first happened as it's infrequent enough not to be a bother really. It's only very recently that on every boot it started taking a long time to complete booting and resyncing the controller whilst doing it but that seems to have been fixed by the latest flash. So the only remaining problem at the moment is that it (once) failed to boot with all four red lights on solid.
I was reading this thread about using transistors for the JTAG http://forums.xbox-scene.com/index.php?showtopic=719652&st=120
and thought that might be relevant although I haven't got my head round it yet. It's been a while since I did the JTAG so I can't recall exactly what I did but it would have been a standard mod following whatever guide was current at the time. I see some nand modding utils refer to AUD_CLAMP but I don't know what that is or if I should tick it or not (I'd generally say not if I don't know what it is but I might just have forgotten, as I say it's been a while!)