engine problems

Actually your V-12 is set up to run as two separate 6 cylinder engines. What I mean is, they have independent engine control modules and with only one side running your vehicle should still be drivable up to 133 mph, when it is running as designed it is supposed to reach 155 mph. There an interface module that coordinates the two halves of the motor to run properly. This is also drive by wire, so you have a pedal sensor and if this is bad it could cause the problem you are experiencing. Best to find an independent shop that has the proper equipment to test these item, because none of them will be cheap. You can try a dealership, but many do not support vehicles that old.