Jag XJ6 - Fault code PO 727 present causing auto trans to go into 'limp' mode. What do you think fault could be?

Most limp modes are caused by transmission faults but can be caused by engine sensors and blown fuses. If your car is in limp mode the fault will be stored in the ECM.
You need to read the fault codes stored in your ECM to know which sensor has caused this problem. You didn't say what model vehicle you have and it would take a number of hours to list all the possible codes and reasons.
Get someone with a code reader for your model vehicle.

Try clearing the code from the computer. If it returns again, the second hand one could be bad also! Some codes are what they call a hard code, that stays in the computer memory, and will think that the problem wasn't repaired (even though the part was replaced). Till the code is cleared from the memory, the vehicle will return to a limp mode untill the problem is resolved!!

Sounds like your vehicle is in limp mode. Check for faults with a fault code scanner at a local auto parts store. Resetting the fault will fix the limp mode, but not whatever caused your vehicle to initiate limp mode.

The on board computer (ECM) has put the car in limp mode (reduced power) because it has detected some kind of serious error in the systems operation, this can be the engine or the electronically controlled transmission, it does this to prevent any further damage to the car engine or transmssion or possible fire hazard in the case of a misfire or rich fuel condition that overheats the catalytic converter. You must now have the computer control module scanned for fault code and repaired based on the diagnostic testing outlined for the code inthe factory diagnostic manual..

What does the code Pro 1 mean on a 1995 jaguar XJ6

Fault code P1178Rich Fuelling Trim, Long Term (AMFR)Check for manifold air leaks, rectify as necessary Check for recorded MAFS fault code, rectify as necessary Clear fault code and perform service drive cycle to verify fault cleared Contact Jaguar Service Hotline if fault still present.

Taken from the Jag service manual.

Check your Throttle

the check engine light will be on 

When this problem happens I would check the timing.  As your question was not fully reproduced, I would appreciate a more detailed description of your problem.  It could be the onboard computer has a fault and you are getting warning lights and the more obvious symptoms as a result.  Michael

Code 24 is a vehicle speed signal sensor fault, the sensor most likely does not provide a useable signal until about 25 MPH, when the computer doesn't see any signal after that speed it turns on the light. Make sense?