Whats wrong with my garage door opener?

I have a 2 year old 1/2hp Liftmaster garage door opener that seems to all of a sudden have a mind of it's own. It goes down a little and then back up, then sometimes it goes all the way down. Sometimes it only goes a few inches then back up. It's driving me nuts. I have the sensors pointing directly at... show more I have a 2 year old 1/2hp Liftmaster garage door opener that seems to all of a sudden have a mind of it's own.
It goes down a little and then back up, then sometimes it goes all the way down.
Sometimes it only goes a few inches then back up.
It's driving me nuts.
I have the sensors pointing directly at each other, and the down force adjustment is set at maximum.
When I unhook the opener from the door, the door moves smoothly.

HELP!!!
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