Actually its probably a broken wheel in your odometer. Its a relative simple job for someone who has that gear in stock. Merc made all their gear in their odometer out of plastic except one which is made of rubber, over time this rubber gear melts and gives way, You can find online these gears onsale. the other thing is to get someone to pull out the meter with a special tool. all in it would take you 20 minutes to uninstall and reinstall if you know what you;re doing., the cost is only for a gear that maybe cost 50 cents but it really depends how much the bloke wants to make from you. dont bother of changing the whole meter cos its not necessary as the replacement is probably secondhand and the rubber gear hasn;t been changed to a plastic one,.

Only draw back is that after changing to the plastic gear your milege might look juttering when it moves as after market gear are not normally perfect fit but it works brillant. PS when u drive u look at the road and speedo meter and not how many mile you pass