engine accelerates by itself then goes to fail safe on 1999 Jaguar XK8

the engine starts and accelerates by itself to 4000 rpm!?
then goes to fail safe mode

Jaguar had some trouble with the throttle body housing. Jaguar do have a bulletin about improper throttle cable adjustment causing problems. IS there any independent Jaguar repair shop in your area that is knowledgeable in the repair of Jaguar. There are few electronic scantools that operate well for diagnosing Jaguars and the ones that work well for Jaguar are expensive.
the shop says it might be throttle body (says it costs 2,500.00 remanuf???!!!) or the computer that controlls it??