We are having a problem with our garage door. Some times it takes several tries to get it to close, and some time the only way to get it to close is hold the button inside of the garage down until it closes.

It is a 18' metal overhead door. The door faces south with no shade. When it does not close it is at times of the day when there is unequal heating across the door, ie in the morning when the sun comes up, and only part of the door is in the sun. Or a similar situation in the evening.

How do I go about trouble shooting the door, so it will open regardless of the sun.