The clock is made largely of acrylic plastic which has limited dimensional stability at 20 plus years old but the most likely causesof problems are going to be dried lubricants and dirt. I would strip the face and hands then ultrasonically clean for half an hour in lukewarm water diluted ten per cent Flash liquid followed by half an hour at five per cent, then ten minutes in plain water. Dry in airing cupboard overnight. Ensure the pole pieces of the magnetic core are clean and there is a gap between it and the rotor. Lubricate each bearing, not the teeth of the wheels nor the leaves of the pinions with the tip of a wooden cocktail
stick.
For the lubricant, a thin engine oil, 5w 30 or 40 will be just fine.
That ought to work.