I have owned my 02 XJ8 VP for about 5 years now and I've noticed (or at least I imagine that I have noticed) that while the rain sensor works, it seems to be a little more erratic in its operation. Sometimes it waits slightly longer than it should to make the next swip. Purely by accident recently, I discovered that, when in the rainsensor mode, I can activate the wipers once by sweeping my hand between the rear view mirror and the windshield without touching anything. It seems to work every time. Not an elegant solution, I know, but it's kinda fun to feel you are beating the system. If you have this 'problem', give it a try!

 

Is the sensor still fully glued to your window? Or has it started to bubble or come off...you can tell by looking at it from outside the car...could also be your wiper relays...

 

I had to change the sticky-goop that holds it to the window on mine, prior to that it if I simply touched the sensor housing it would make it wipe.