The casing may have become warped over time, and hence the pendulum is now running out of true. Try lifting the clock on one side (or the other) by placing a thin wad of cardboard beneath the feet. You may need to have the clock permanently tilted to one side or the other (or leaning slightly forward or back) to allow the pendulum to swing freely and evenly.
Dean Felton - Melbourne, Australia, April 2006