Most likely suspect is the odometer gear.

They are available from www.odometergears.com There are other suppliers selling the gears now and some have received less than positive reviews.

Pictorial instruction available on Art's page at www.cleanflametrap.com

You can search the site through the search window at the top of the page for additional information.

The gears get old and a tooth that has broken off is usually the cause. There are actually two gears that can be broken, but my experience (3 times) has been the smaller gear is at fault. When you open it up to check on what you need be on the look out for the broken tooth so it doesn't stay behind to cause problems after reassembly.

Randy