Sturmey Archer gearbox issues

[Oily thumb nail]

Still struggling to sort out a few issues with my 1930 R9.

The main one is the gearbox. I worked through the linkages from the gear leaver to the box, taking out slack resulting from wear in the cotter pins and gear leaver. After some fine adjustment I can now select 1 to 3 and neural OK. The big problem is 2nd. It selects fine but when selected on the road and under load it produces noises from the gearbox which sound like its partly jumping out of gear but the gear leaver stays put in its gate! I have put the bike back in the garage not wanting to do any damage.

I have now reconciled myself to the fact that I am going to have to take the gearbox out. Any suggestions what might be the issue and what to look for in the gearbox other than obvious wear. What is parts availability like for Sturmey Archer gearboxes that are 92 years old! Where to go for parts?

Any sensible and helpful suggestions before I get the spanners out and start work.