This happened to me. With a little research I found out that there are little plastic things in the agitator that get worn over time. I think they were called feed dogs. It only required taking apart the agitator and replacing the dogs. It was a very long time ago so I don't know what the parts cost but I think it was less than $20 and I did it myself without my husbands help. I bet there is a youtube video
There is a transmission that changes the motion of the drum. A relay or switch can be faulty, Broken spring, all sorts of things. The best remedy is to have a pro check it out to determine what the issue is. If its transmission issue, the cost of the repair may be more than what a new one would cost to purchase. Relays or switches can be replaced fast and are not all that expensive.