Sligh grandfather clock is fast

by Paul
(Atlanta, GA)

I have a Sligh grandfather clock that is about 20 years old, model 0830-1-an, movement 8000.
My hour hand is out of sync with the minute hand. Over the course of a week the hour hand will gain about 15 minutes over the minute hand. I have just been moving the hour hand back a little every week to compensate but it seems to be getting worse.

I had the clock cleaned and oiled a few months ago but this didn't correct the problem.

Any suggestions?
Paul