I have a Jaguar XJ6 3.0 when I cruise at 35 - 40 mph the revs go up an down the engine light comes on. What is it?

this sounds like a torque converter issue.... this was a common issue with these and some xj models between 2003-2005...the symptoms will be the car would rev while cruising at a certain speed...but the speed wouldnt increase do to slipping...

Do you have cruise control,see if that cable somehow is giving slack,This is a problem with Hyundai where the cable affects TPS plausibility,There was a TSB bulletin by Hyundai Engine RPM fluctuation - cruise control cable adjustment procedure

have check engine light scanned for the problem before you damage engine [ overheasting or transmission fault ] and your overdrive is possible connected to you over heating check engine so get vehicle scanned code will assist in correcting problem

Your dealership should have seen any underlying codes. Most Cruise controls I've worked on are used on the vehicles vacuum. that is where i would start, because on a grade your engine uses more vacuum pressure thus taking from some where else.

hi i had this problem with engine management light and no power over 2000 revs the solution was replacing split breather pipe and all codes checked and cleared, the cost was £30 @ jagtech stockton hope this helps

Have the codes read this will shed some light on the situation,it can be numerous things that can cause the problem you are having.Also have the alternator checked.

I saw exact same problem for few days. Now I see check engine light all the time and engine speed is normal.
The cruise control stopped working. Cruise control sign so nOT light up when I turn it ON. Nor does it works.
But car seems to work fine.

I have a 2002 X-type Jaguar and the check engine light and cruise control unavailable comes on but the cruise control light goes off once the car warms up. Any ideas?

check your cruise control unit first

never heard of this before ,so obviously the cruise control is allowing the throttle to work then when it has a cough it puts engine on limp or slow speed .this vehicle is fly by wire so try changing the sensors on the pedals .Or just put it into dealers because if the correct part isnt known then changing parts willy nilly could cost more than a dealer repair