It is now stable at 390/350, and the memory can be slightly overclocked to 385MHz or so. The core is unstable even with 5 MHz overclock. I'm told it was flashed with a non-original BIOS, so maybe that's why, or the core was already unstable, and delidding was a last-ditch effort.

This is absolutely brilliant. Congratulations!

Okay, so it was originally an FX 5900 ZT with stock clocks of 325 core, 350 memory. That explains the limit of 390 MHz for the core, it probably needs a volt mod for more...