The problem was the PSU. When I took it out of the case and watched the system boot. During boot, a few seconds after startup, the PSU fan would stop and start again. The PSU Zalman ZM600-HP 600W was from a working PC that had an ASUS MB with an Intel Quad processor and was consuming much more load than the current MB. I connected the PSU Dell NSP-525AB and all problems were solved.