My old Craftsman 1/2 hp garage door opener was abused by previous owners who tried to operate it with a poorly adjusted door. Now, the door operates smoothly and the opener travel limits have been properly adjusted along with the safety stop. The problem now appears to be that the door closed limit doesn't seem to operate at the same place each time. It is a chain drive and sometimes when the door closes you can tell the stop limit has not operated because you can see slack in the chain just before the torque safety switch causes the door to open. At other times the door doesn't close far enough to compress the rubber door seal.
The opener does have up/down force adjustments and they work fine. The problem seems to be related to unreliability of the down travel limit adjustment. There has been some unreliability in the up travel limit too, but the primary problem is down. After each adjustment it may work fine 2 to 5 times before it again closes too far stretching one side of the chain very tight and causing the bar to bow, (unless I properly adjust the down force safety in which case the door reopens). I plan to open it up and look for anything loose, like chain sprocket to shaft or loose limit actuators. If I don't find anything I will take the advice below and buy another opener.
Best Answer
Many times the most cost effective and time effective (and "time is money" cost effective) solution to strange problems with an old opener is a new opener.
You can, of course, replace parts; but you can quickly exceed the cost of a new opener, and parts often come with no guarantee, while a new opener generally has one. Since you KNOW your opener has been abused, that's another tick in the "reasons to just replace it" column. Recall that it is a device that can kill people if it malfunctions in certain ways.
I will also suggest (from experience) that you provide a surge suppressor on the outlet serving the opener.