Garage Door Won't Go Down

I have a Chamberlain 1/2 hp garage door opener. I have an intermittent problem in that it won't go down about 75% of the time. Now if I hold the button on the wall switch I can force it to go down. When it doesn't go down, it will do down about 1 to 3 feet and then stop and reverse. The warning light flashes. It acts like something has blocked the eye sensors but nothing is in the way. I tested the sensors and they are both on and
when I stick my hand between them the lights go out so I...

Read more