Vauxhall Astra G wiper fault?

Hi, hopefully just an easy one to know but, my 2000 X-reg Astra front wiper blades have start to just carry on goin after you knock them off, if you just try and turn them to intermittent they just work on constant and then carry on working after you've switched them off for at least another 15-20 wipes then suddenly randomly remember to stop wiping!! Anyone know whats causing this, motor, replay possibly????