Vanden Plas traction control

1999 Vanden Plas V8 XJ8
I can be driving along and the traction contol failure message comes on and the engine goes into a engine fail safe mode which allows the car to stay on and move at an idyle speed. Does not allow for acceleration. It happens intermitently and it can be first thing in the morning or anytime at all. Happens with no notice and no pattern. Dealer can not find any problem. I can turn the car off when it goes into this fail safe mode and it then most of the time resets and I can either drive some more or it triggers the same and without notice goes into the mode once again which leaves me in traffic shutting down the vehicle and starting it up once more. This is very dangerous and jaguar should care enough to. Address. The car has 73,000 miles and this has been going on since 40,000 miles. Please help me.