Does the trolley track or rollers need lubrication? Are the rollers in good shape? You can also make small adjustments to the sensitivity of the opener. Your manual should tell you how.

I had the same problem. I found that the "T" shaped bar that the carrier (trolly) runs on tends to get slightly misaligned in cold weather, thus causing binds. It is made of two pieces that are bolted together halfway down the bar. I found I could smack it back in place with a hammer, then tighten the bolts. Then it would work for awhile longer. (The newer models have reshaped that bar to be square and the ends fit together better.)
Make sure all rollers and hinges on the door are lubricated.
Another thought: you said it starts and then reverses. I took that to mean that it goes part way. But...If it is closed, and starts to open, but doesn't actually lift before reversing, then your door might be frozen with ice at the bottom. I've had that happen.

[This message has been edited by rpxlpx (edited February 26, 2001).]