I suspect that your memory settings are all on AUTO which means that the memory is being overclocked past its ratings.
Download CPUZ and check your FSB:RAM ratio. It should be 1:1. At a stock FSB freq of 266 MHz, your memory frequency should be at 266 MHz (the way CPUZ calculates it) and your memory clock should be at 533 MHz.
With a core speed of 2.91 GHz, your FSB is 388 MHz. If your memory settings are on Auto, your memory clock is actually (388 MHz/266 MHz)*800 MHz or more than 1100 MHz.