I have had one dually (a Tiger MP system) that has had both of the L5 mods go bad (at different times.) Both times, I had to troubleshoot to find out which CPU was having problems, and both times, I had to remod the CPU (more than once, to get it working right.) I don't know why this one system keeps giving me trouble, when my four other dual AMD systems don't, but there it is.
Mods CAN go bad. When they do, you have to fix them. In my case, the machine in question is only used a couple hours a day, so it wasn't a big deal to fix it and no one even noticed the problem, but me. On the Tiger MP, if one CPU starts reading as an XP, the system will take about 75 seconds to POST, but after that, it will run fine, although with just one CPU.