2000 Jaguar S-Type 4th to 5th Gear Jumping Back and Forth

The brake switch fault could be the cause of the transmission shifting error. Repalce the switch and repost with your results. Check for any engine vacuum line leaks for the idleing fault !

Did you delete the fault codes from the cars computer?. They need to be deleted and the car go through a relearn drivecyle. If you did delete the codes is the check engine light back on? Is there any new/same codes? Did you check for vacuum line leaks?

Note : not all generic handheld fault code readers wil supply transmission codes. Have the vehicle plugged into a professional code reader. (Autozone/aamco test free usually)

Sorry I did not get back to you sooner work/family commitments etc.

A cylinder misfire will not generally cause erratic shifting between 4 & 5 ... did the codes come back ? is it still shifting erratic or harsh between any gears ?? .. there are numerous PCM (computer) reflash updates for this vehicle it could be it has not had them all ? I would not advise a transmission flush on this vehicle !!

Let me know

Yes, i'm afraid only dealers in the U.S. have the software to reflash the computer ... I have been thinking about your idle problem and remebered this TSB !

POOR IDLE/CHECK ENGINE MIL - V6 ENGINES - IMPROVED BREATHER ELBOW - SERVICE ACTION S158
TECHNICAL SERVICE BULLETIN
Reference Number(s): S303-S158, Date of Issue: July 2003
JAGUAR:2000-01 MY, S-TYPE 3.0L
VIN:L00001-L86901
SECTION:Engine System

Related Ref Number(s): S303-S158
ARTICLE BEGINNING
Issue
S-TYPE 3.0 liter V6 vehicles, within the above VIN range, are equipped with a 3/8-inch bore 90 ° rubber elbow as part of the engine breather system. This elbow is manufactured from a type of rubber that is subject to degradation from the gas and vapor that pass through it, resulting, after a period of time, in air being allowed to enter the intake system of the engine.
Vehicles within the above VIN range may exhibit poor idle and a 'check engine' MIL may be displayed on the instrument cluster.
Action
On 2000-01 S-TYPE 3.0L vehicles within the above VIN range, install a revised breather elbow, at the next available opportunity.

Also remembered there was an updated valve body introduced for inside the transmission on this year vehicle

5-SPEED TRANSMISSION - ERRATIC OR HARSH SHIFTS - VALVE BODY KIT AVAILABLE - REPAIR PROCEDURE
TECHNICAL SERVICE BULLETIN
Reference Number(s): S307-19, Date of Issue: June 2005
JAGUAR:2000-02 MY S-TYPE
VIN:L00001-M44997
SECTION:Transmission System

Related Ref Number(s): S307-19
ARTICLE BEGINNING
Issue
Some 2000-02 MY S-TYPE vehicles within the above VIN range may exhibit erratic or harsh transmission shifts. This can be caused by valve body or control solenoid failure.
Action
NOTE:Check for connector/wiring and Powertrain Control Module (PCM) faults, ensure the transmission fluid is clean and there is no debris in the sump. Refer to Global Technical Reference GTR Workshop Manual, section: 307-01 - Diagnosis and Testing. Any faults other than the valve body kit replacement, are to be carried out as a separate Warranty Claim.

When a fault has been confirmed with the valve body or control solenoid, a new valve body kit must be installed. Follow the Service Instruction.

check/replace the elbow connector and test vehicle .. although i'm not sure both faults are connected .. check to see if both are still occuring ?

let me know