While driving on the highway (4 lane divided U.S.) my MY 99 XK8 went into safe mode as I slowed to merge on to the exit ramp. There was a hard downshift followed by the following additional indications on the dash panel. Traction Control Failure, Restricted Performance, Stability Control Failure, Transmission Fault, Fail safe engine mode. When I got the car home I retrieved the following codes: Air Related - P0102 (Mass or volume air flow circuit low input), Intake Air Temperature -P0112 (Intake Air Temperature sensor 1 circuit low input), Throttle Body - P1121 (Accelerator demand sensor A circuit range/performance). Went through all the pinpoint tests identified in On-Board Diagnostics Manual and Electrical Guide. All checked out OK. Both Service Bulletins 303-5498 (not needed) & 303-58 (Connector Material Mismatch) had been worked several years ago. I worked through all of the pinpoint tests referenced in the Jaguar On-Board Diagnostics Manual and found all tests OK. 
( I visually checked the throttle cable at the throttle position sensor and found iit slightly soft. It wasn’t droopy, just soft when pressed with a finger. I adjusted the throttle cable at the throttle position sensor at the throttle body 1 turn tighten. Erased all error codes. Car has operated normally. I hope this is the answer. 
Same year XK8 …same code…chase all the same “fixes”. Do yourself a favor…replace the throttle body. Rebuilt ones are available…less than $600…takes less than hour. Cheers!
I was hoping Dan’s recommendation was not going to be the final resolution to my problem with the throttle, but, as it turns out, the problem returned in August. I replaced the throttle body last year and the car has been running perfectly since. Thank-you Dan for the recommendation.
Mine just started acting up, and the throttle seems like it sticks and the car will run at 35 MPH all by itself, and it just may be the slack on the cable that is causing this. It settles down when you stop back to the idle rpm.
You need to post the actual DTCs logged in the ECM.
Make Model Year will also help the diagnosis.(there are different variations of XK8)