Hi, I'm not that technical, so you can ignore my post. However, some thoughts:
-according to ASUS, the CPU is supported starting with the 0905 BIOS version;
-after replacing the CPU it would be good to clear the CMOS;
-again according to ASUS (memory specs for the board): 4 x DIMM, Max. 8 GB, DDR2 667/533 ECC,Non-ECC,Un-buffered Memory Dual Channel memory architecture Native DDR2 800 Support
So, although DDR2 800 advertised, I would try though testing the Q9550 with lower freq memory (667 or 533). Just for the sake of it. After all, it might be a BIOS issue.