I have a six month old Brompton 6 speed with the two speed Sturmey-Archer (I believe) rear hub and 3 speed rear derailleur. Yesterday it developed a problem for the first time.

When riding, if I put any significant pressure on the pedals, it felt as if the drivetrain was "skipping" gear teeth or something. There was a small "crunch" type feeling/sound, and the pedals would jerk ahead a small amount. It would do this repeatedly for so long as I tried to pedal at all hard.

I'm pretty sure the problem is in the two speed rear hub, as I did my best to watch the external gears of the three speed derailleur and didn't see anything untoward happening.

At first it seemed to do it only in the higher two-speed gear, so I kept it in the lower and shifted only with the derailleur, but later, with much pedal pressure, it also did it in the lower gear.

Then, strangely, when I folded the bike after my ride the chain got all slack and droopy, and it dropped off the lower derailleur cog, the one that is on the longer arm than the other. It's never done that before - is it related to the problem above?

Any suggestions as to what a home mechanic might try to troubleshoot and fix?

Although I imagine that the warranty may still apply, I bought the bike from a shop 2 1/2 hours away so I can't easily take it to them. There is a small but good LBS that I have a good customer relationship with, but I don't imagine that they could do Brompton warranty work because they don't carry them. They might be willing to work on it, but I've never seen them carry an internally geared bike in their shop.

Thanks in advance.