Can't find in manual "ASC' trouble

ASC is short for "automatic stability control" u will need to have the ABS/Traction control system tested by the dealer for a possiable fault code.

For any warning lights, (1) hook up an OBD-II electronic code reader (~$40) to the connector (on XK8, just left of the driver's left knee, very near the driver's door hinge. Follow its directions and you will read out any engine trouble codes.

I have not seen "ASC" but I have seen "ABS" show up. If the trouble is in the ABS braking system, that needs a special code reader which attaches inside the engine compartment and reads out the ABS electronics system. A Jag dealer or specialized service shop will have that reader. (On XK8s, the ABS electrical control board commonly develops an intermittent solder connection...and an "ABS" error and possibly "Tracking Disabled" may be displayed).

Best regards,
Michael