Idle jump, car lurching at idle.

Hey everyone,

Here I come again with yet another question :(

My 2000 XJ will idle normally for about 10 seconds, followed by a 1 to 2 second "lurch" in RPMs up to 1k RPM, then settle back down to normal. It will repeat this process. It also may be my imagination, but it feels like when I am braking the problem is more pronounced? I'm not sure though. Has anyone else dealt with this? I assume it's some sort of vacuum leak? Any insight would be appreciated. I am also getting code p0456, but I don't believe they're related, as that's the EVAP system. I am probably going to have a shop smoke test it since I can clear the code and it'll take a week or two of normal driving for it to show up again (a very, very small leak I suppose).

Check all vacuum lines and replace those that will not stay on their fittings, or those that may be dry rotted/brittle.

Check the step motor and clean it.
That may help.
(I think this part name in English is step motor, I refer to the part near the TPS which control the idling)
Saar.

Thanks for the tips, I will take a look at those.

I believe I found the issue. The rubber elbow that connects the vacuum line from the LDP to the manifold is dry rotted. I literally removed it from the intake nipple with my pinky. Any idea what size that elbow is, or a part number? Thanks.