Airburst

Basically, what it says in the title. I tore the whole thing down completely when I converted it to oil lubrication, and it was fine before and after that, and none of the internals were visibly worn or damaged. A few days ago, on my way to work, I felt a strange sort of grinding sensation through the pedals, which got me worried, but then went away after ten pedal strokes or so. On the way home, however, there was an audible "crunch" as I pulled away in first, and the sensation returned. It went away again, but today, it started to get REALLY serious. The grinding is now constant, and at one point, it slipped slightly, and when I started cranking hard away from a set of traffic lights, there was quite a loud cracking sound and another small slip.

All these issues are only happening with first gear. It's definitely not the cable adjustment, as I've checked that, and it's exactly the same as it has been for the last few months. Before I take the whole kit and kaboodle to pieces again, has anyone else had this issue, and if so, what was causing it?

For the Sturmey-Archer gurus out there, I suspect the gear ring pawls may only be being partially depressed by the clutch correctly when the hub engages first, so basically, it's trying to drive itself in second, but the pawls can't quite manage it due to the fact that they're being half pinned-down by the clutch, so they skip with a "CRUNCH" and the hub stays in first. Why on Earth that would happen is another matter, but it's all I can think of until I strip the hub down on Saturday.

Edit: I stripped the hub down, all I could find that was wrong with it was a broken pawl spring on the gear ring. That shouldn't have been causing the issue, but I haven't ridden the hub since, because I also discovered that the rim has a massive crack in it. It could be that the extra torque of first gear was causing some sort of variation in the spoke tension, opening the crack further and producing the grinding sensation, but that wouldn't explain the skipping.