Winscreen Wipers Help?

I brought a Corsa N reg the problem is when the windscreen wipers are in the "timed interval wipe" setting it doesn't work properly its stops at random places then continues and stops again.

Also when the windscreen wipers are working when I switch them off it doesn't stop at the bottom off the screen its just stops at a random place.

This is really annoying and distracting when driving.

Can someone help me please?

Thanks in advance

Steve
March 2006