It could be many things, but begin with the electrics as earth problems cause many such occurences.

Clean the motor, often they get clogged with dirt over time, as well as carbon dust from the brushes. Check the brushes also, if they are worn it will give these symptoms, and an auto electrician will order you a set. They are cheap, and easily installed.

Check the gearbox, remove the cover and ensure the bearings are free and it is filled with clean grease. Over the years they become contaminated with all sorts of rubbish which clogs them as it mixes with the grease.

Check the wiper spindle, often they become dry which causes them to bind, or their inner bearing surface wears to an oval. If it is binding a little plastic or rubber lubricant often works.
If you strip the spindle itself them lubricate with a little silicone grease.