Engine lack power
 

My 2002 x type jag not going beyond 3000 rpm and idles rough

Hi,
I am assuming it is a petrol engined X type.
Check the breather pipe that runs from the cam cover to the inlet manifold as it usually splits and causes an air leak which is too much for the ecm to compensate for and to prevent engine damage it reduces power at @ 3000 rpm (feels quite violent when 3000 rpm is reached).

Hope this helps.
Gary