Vauxhall Vectra front window wipers

Have had a recurrent problem with my front wipers initally thought it was seixed up linkage arm as they would work then stop mid flow had the arm repalced but the problem reuccured a few weeks later now have been avoiding the intermittant selection on the wipers as it seems to begin if they are set on that but was agian caught out late last night by them in the dark and pouring rain with my boys in the car eventually got them going again which i do by getting out of the car and giving the wiper a tug cannot carry on through the wet winter months like this need help does anyone know what the problem may be ? thanx t