Do I need to replace Garage Door Opener or am I missing something obvious?

The garage door opener intermittently stops working and the problem is getting worse over time. When opening, it starts to go up and then stops and the overhead light stops illuminating. Sometimes, I can slap the side of the unit with my hand, the light turns back on and the door will work again. I suspect either an intermittent short circuit or a loose electrical connection. I have played with the closing and opening force, and am certain misalignment of the garage door sensors on the floor have nothing to do with the problem. Is there a weak point that I can easily check and fix, or do I just buy & install a new garage door opener?