Jumping out of second gear

[Bretty]

The dogs will be worn where the gears engage side by side. The dogs are nice and square but each time it jumps out it'll wear a chamfer on the edges, it gets worse and worse until they no longer stay engaged under any torque... As it jumps out it'll likely be bending your new fork. :-S

You need a new gear box! I'll take a photo of my knackered gears, if it makes it clearer...