Work off this list of compatible RAM -> http://www.corsair.com/configurator/product_results.aspx?id=688452
The CAS 4 probably never was the correct RAM for your ASUS P5Q.
1GB 800 4-4-4-12 @ 2.1v
2GB 800 5-5-5-18 @ 1.8v
XGB 800 5-5-5-18 @ 2.0v <= You have 3 bad things: CAS Timings, Voltage and Size differences which do NOT work well together. Therefore, you're NOT Solved - you're instead 'creating' a Problem.