UK Car Repair: XJ8 Idle problems, vacuum leaks, maf sensor

Question
I have recently brought a 1998, 86,000 mile, Jag XJ8, 3.2 sport.  The car drives and runs really good.  The only problem is when i slow down at the lights or come to a stop and put it into P.  It will carry on running but it will idle at 650 and 800 going up and down at a rythm at the same time you can actually feel the car/engine making an up down motion.  when you put it into D you then feel the whole car moving up and down, but when you take off it runs so smoothly and beautifully just like a Jag should.  There is no smoke or anything coming out of the exhaust and the engine oil level is good and then it drive real good too.  I just dont know what it could be.  I have it booked in at Jaguar Garage next week but i dont want to get ripped off or be told the engine is a bad one and it will need replacing.  Please can you help is it something little or will this cost me a fortune?  Please advise?

Thanks

Jagjit

Answer
The problem is not the engine, it is somewhere in the control system,  Perhaps the throttle body should be cleaned first, this has caused these kind of problems, then check for vacuum leaks etc. Otherwise it is likely to be the MAF sensor on the intake hose or the throttle body its self