More than likely your window motors are going out. The only way to properly diagnose them is to take the door panel off, find the connector going to the motor, backprobe BOTH wires at the same time with a 12v trest light or similar while the window switch is pushed. What you're doing is making sure that there is both power and ground at the motor (just checking for power isn't enough). If there is power and ground at the motor when you push the switch up and down, then you have a bad motor. If not, let us know what you found and you can go from there.
BTW, both of my back windows have the same problem and I'm too cheap to fix 'em (I did fix the fronts). So I just took the cover off of the window switches for the rear ones so no one would accidentally let them down.