my car did the same thing, it would work sometimes and if it didn't i could press on the side of the door panel to get it to work. i thought it was the window motor so i got one off a friend and it didn't help. It turned out to be the window regulator(little black box that the switch plugs in to). i got mine from a junkyard for free, along with a nismo power brace and front lip for $20 bucks...those country folks know nothing about 240's.
http://www.nicoclub.com/FSM/240sx/1989-90/POWER_WI.pdf