Bike is an '04 280 Pro, runs fine, not had it too long but had it in some quite steep positions yesterday and it would tend to stall when held with the nose down on a steep hill.

Any ideas?

My guess,float level slightly low/high?

Sounds like floats are set wrong. I assume you are on the rear brake when descending. Make sure you are pulling in the clutch so the brake is not stopping the engine from running.

To be honest I think its stalling more when I'm doing stoppies down hill. I do have a bit of clutch drag so thats not going to help if I'm on the rear brake, but I think the clutch plate thicknesses would need looking at Iin order to dial out the drag.