There is no way to tell what the problem is, but it sounds more like old oil gumming things up and it could use a good cleaning and some fresh oil, a special oil that is very thin, much like sewing machine oil, but thinner, yet.
While I don't know your exact model, I do own a number of Seth Thomas clocks. To get it fixed can be easy if you know a watchmaker. Try your local jeweler, but that can be expensive. If you don't know someone, it can cost and easy $75 just to open the thing and clean it. These old movements are quite basic and are easy to fix, provided parts are available, if necessary.
Unless this is some wild rare clock, it's probably in the $250 range, reasonable retail.