As kickaha says above...
A common cause is the dogs on the gears get a rounded / taperred edge so that when the load comes on the dog pushes out of the slot and you get it jumping out of gear.
see here - but the zook ones normally have a flat face not like the pegs in the picture.
http://images.google.co.nz/imgres?im...%3Den%26sa%3DN
What can be done reasonably easily with the right gear - phone around - is you mount the offending gear in a lathe chuck and a grinder in the post. Then the angle on the driving face of the dog gets reground.
Sometimes the angle is even cut negative rather than flat, but thats just silly.
The other option is that the slots that the dogs fit into are rounded off - much harder to machine these - I've seen a butcher job done years ago with a dremmel. Its often just as easy to get a new cog if that is the case.1
Here is a cog from the mighty SV. The arrow is pointing at the dog face.
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