I added a second CPU to my Mac Pro 3,1 (Early 2008 2.8 mhz) and now it hangs at bootup. Purchased a E5462 slant Xeon Quad Core 2.8 mHz, a heat sink to replace dummy heat sink, and thermal compound. First time I started it with added CPU, took about 10 minutes before I got the Mac Startup sound. Thought maybe it needed to some internal checks with the new CPU. The computer ran fine, showed the two CPU's thought all was good. The mac went into sleep mode and when I tried to wake, it stalled again like the startup. Did a cold re-boot and same thing at startup, just hung. Is it possible I didn't screw down the heatsink hard enough to new CPU? Right when I felt some resistance, I stopped, didn't want to over-tighten.