Hi das_stig, thanks for the comment.
I found this thread, it appears to be the exact same problem,http://www.tomshardware.com/forum/247629-28-suddenly-slow-full-speed#t1780716
So I thought the problem might be that the fan was not working and thus the cpu was underclocked.
I installed i8kfangui, (a program to monitor temps and control fan speed)
But when set manually to full speed it appears to be working fine and temp seems normal. Is it possible the fan is still working but its just not being activated by the motherboard?
I tried disabling Intel speedstep like in the above thread and it increased each core speed to 800MHz (up from 600 but still far too low). New experience index scores are 3.1 for CPU and 4.3 for RAM.
It seems like I may indeed need to call Dell,
however my warrenty is up.
It is only covered for the well known problem with the 8400gs GPU and I'm on my 3rd motherboard due to that problem!
Does anybody have any suggestions as to how I might fix this myself?
All comments are very much appreciated.