Slipping gear on Flying Scotsman!

[phantom892]

I recently got a second hand Hornby Flying Scotsman loco. It is all in super condition, but when it runs it suddenly stops - as though gone/slipped out of gear.
When i try the wheels, they turn freely with nothing engaged internally. If i wiggle them side to side slightly, the gearing re-engages and it then runs again for a short while before slipping out of gear again (or at least i assume that is what is happening).

Is this easy to check out? I have small tools, etc, and am able to work on machines - but know nothing about the workings of these locos and don't want to start to take the motor apart and suddenly loads of springs or items come flying out!

Any help with how to check the gearing appreciated.

Mark.