window motors

i have a 1991 lesabre 4 door both front windows did not work. after reading on this forum i went out and got a pair of pontiac transvan motors they work great with the window not hooked up, but after installing them in the doors they go down and up but they stop must wait a few minutes or i have to pull up on the windows for them to work. could they be bindding up or something
thanks
jenn