Could be a bunch of different things. CPU can't handle it, memory controller crapping out, memory timings need tweaking. You have to find the source of the problem.
First of all, to use SP2004 correctly, use the custom test. Set the minimum FFT size to 8 and max to 4096. Uncheck "Run FFTs in-place" and set the memory to use to about 700-750MB so it doesn't use the page file.
If it can't pass 16+ (24+ recommended) hours without error, then your computer is not stable.
You can use Memtest86+ off a floppy, and if errors, chances are it's the RAM / memory controller, but I've seen MemTest86+ go on for hours without error, even while unstable.
1.58v. That's a lot of voltage. What are your load temps? Considering backing down a little bit. You're putting that CPU at a lot of risk with that kind of voltage.
Good luck. :beer: