Update Hi All I have spent most of this weekend experimenting with this system to try and understand the CPU throttling issue. So far it appears that everything is working perfectly. The CPU frequency scaling feature in Linux is working as it should do and does not seem to be limiting performance. The Intel turbo boost is working and the CPUs are throttling to stay within their design parameters. The difference between the two chips could simply be due to variations in manufacture. One is running at about 2220MHz and the other at about 2340MHz. I have been running with the slow CPU in socket 0 and the fast CPU in socket 1. I tried swapping the processors round to see if that has any effect but the system will not post with the faster CPU in socket 0. I think building a multi-socket computer using ES chips from Ebay is not straightforward . It looks like the only way I am going to get more speed is to overclock the base clock. Wish me luck.