My 2000 xjr is giving me the same problems. The technician said my K&N air filter was causing the problem. The dealer replaced it with a "factory" air filter for $65.00! It sounds like BS to me but I'm desperate to get the problem solved! I'm going to check the vacuum lines as suggested.

 

You would not be the first to report problems with the K&N filter!

 

hello there, once i watched this video and idea struck me. maybe this is some how connected with ur issue? i know this is a different car, but still...
check out this link

 

For what it's worth - I also had the restricted performance issue on my '01 XJ8. Strangely enough, this happened just after I installed K&N filter. BUT, turned out it was not related - or at least the code I was able to get was knock sensor malfunction on Bank B. $25 and 20min later, replaced knock sensor and problem never re-appeared (and still gladly have the K&N fitted!).

Cheers