I think it is the lub or the force limiter. Check to see it things are relatively clean. If not clean as best you can and re-lub. I use lithium grease. Next the force limiters. If that doesn't work, I call a garage door company and they will come out. Generally a simple charge is under $100. They also know what they are doing and I think that's important.
The last time they came, they installed a new tension spring and fixed a cable that had "broke". I replaced the garage door opener about a year or so ago with a new one from Sears. They do installations for a fair price. The gear had stripped and it was old, so I replaced the whole damn thing. The garage door gets opened so many times a day, that when it doesn't work properly, it is a huge deal at my house.
