in the jockey pulley near the cog you will see a derailer like stuff connected to the shifters. i guess your problem is you have adjusted the tension of the cable too loose so when the shifter gone to lower pulley it overshoots.

if this is correct, then you might want to check the spacers, but AFAIK the spacers normally goes outside, inside is the axle and the bolt. so this might not be the case.

i will take a picture of the part on my brompton when i got home to show you what i mean.