First thing is to disconnect the drive latch, and verify that the springs aren't worn too much, or are no longer strong enough. Repeated paint jobs really do add up, not counting the springs getting old. You should be able to have the door sit without moving when it is about 1/2-way down if you let go. If it falls, and you still have some adjustment on the springs, tighten them up to compensate.
Another thing you can try if you have a bathroom scale, is to see how much the door weighs - with the drive latch disconnected, hold the door up with some clamps, disconnect the springs, then, with the help of someone else, lower the door down onto the scale. The springs are color-coded - make sure the ones that are there are proper. If they are, and they can't be adjusted to hold the door, buy new ones. If they're the wrong ones, don't try to stretch them more than allowed as that makes them more susceptable to breaking. If they're old, they may not have the safety cable threaded through them to contain them if they break. A breaking door spring can kill you! Don't deal with it if you don't understand or are uncomfortable about it...pay a pro. Some things just make a mess if you do it wrong...but this can kill you.