CPUs rarely cause problems in calculations all on their own, and for the problem to persist despite replacing the CPU, I'd strongly suspect the problem lies elsewhere. Memory, motherboards, and power supplies are all common culprits for causing problems. I'd check the RAM with a day's worth of running Memtest86+. If the memory comes up all clean, then perhaps swapping out the PSU may help. I've had a PSU problem that manifested itself as only rarely causing the CPU to give an error when doing some calculations with SSE FP math. Using standard x87 led to no errors. The PSU died all of a sudden and then when I replaced it, the exact same CPU could now successfully do days and days' worth of calculations using SSE FP math with nary an error. I guess the old PSU had just enough of a voltage droop or ripple to cause the CPU to have very subtle errors that were not severe enough to hang the system, but enough to cause the calculations to fail.