The '99 may be different, but the older XJ's delay control isn't in the wiper motor. Its in a small box that mounts with velcro to the front of the lower dash trim panel on the driver's side. Its a small box, app. 1 1/2 X 2 X 3 (working from memory here), with a wiring harness plugged in on each end of it. I'll practically guarantee that this small box is your problem, as it houses all the electronics for the intermittent function.
4-wheeling: because football, basketball, baseball, and golf only require one ball!
Confidence: The feeling you get just before you fully understand the situation!