I have several watches that run fine for a while, then stop, long before the mainspring has wound down. What usually causes this? Do the watches just need cleaning or is this an indication of more serious mechanical problems?

It's good to have some idea of the probable scope of a project before starting into it.

Thanks in advance for your comments.

In my experience a weak mainspring and/or old oil and dirt mixed in a movement causes watches to run and stop early. The fact they run at all suggests (usually) that with a good service they are restorable.