prime95 question

Hi so I've been trying to overclock my 5000+ black edition. I'm using the ASUS M2N SLI Deluxe motherboard, with ADATA ram, almost the same setup as the one reviewed in toms hardware. So I've raised the multiplier up to 15x and I ram prime95. It gives me illegal sumout errors on one of the worker threads and the other one is still running fine after two hours. the first one says 0 errors and 100 warnings however, so i'm guessing that the system is unstable? but my setup is the same as the one in the review and its supposed to go to 15.5x multiplier without touching the voltage easily. is something wrong?