I have a 2004 Jaguar XJ Vanden Plas with the 4.2-liter N/A AJV8. Sometimes, when I'm at speed (35 mph) and I press the accelerator further, the engine complies, but starts to oscillate a couple hundred RPMs. It's enough to be audible, and it causes me to back off every time. What could this possibly be? A failing throttle-position sensor?

 

Maybe a bad Torque Converter?
Is it worse when cold or warm?
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A failing TPS would initiate the engine light and RP.

Mine does it slightly but only when cold.

We are told it's due to the design of the torque converter blades.

 

Sounds like transmission and not your TPS. Has the fluid ever been replaced? If not do it

 

my XK and the X350 have that feeling when cold. Sort of loppy.

 

I'm not sure. I will have to observe. If I had to guess, I would say when it's cold.

 

Early cars had this syndrome, like my 2003 XJ6 (bought 2010 @30k miles)). It only occurs when the auto-box fluid is cold/luke warm. Once the fluid is warm the rev hunting goes away. I was told it was a clever wheeze from some brainless individual somewhere in the Jaguar empire to speed up the warming of the fluid by locking/unlocking the torque converter. My second XJ6, a 2007, (bought 2016 on 50k miles0, didn't have this problem, so somewhere between 2003 and my 2007 cars the problem was fixed. I was also told the lockup clutch had the friction pads re-profiled so owners wouldn't notice. Whether all this is true or not, when I part-exed my 2003 in 2016, the hunting was still there. I just used to put the transmission into 5 or 4 rather than Drive. Of course if it is there all the time when the car is running, then something else may be wrong. A change of torque converter normally cures the problem.