In late December 2014 I thought I’d treat the chopper to a replacement chain, which I ordered and fitted .
Unfortunately I didn’t get chance to test it right away and ditched the old chain. When I did get around to testing it I found it jumping horrendously making it unrideable 😦
To cut a long story short, I tried different new chains, different cassettes, jockey wheels as well as many other tweaks until I finally had to admit defeat, something I’ve NEVER had to do with any bike mechanical issue.
Then one wet Sunday afternoon I was representing Sky Riders (British Cycling) and I bumped into Rob from the Cycle Centre in Byker who, when I explained the predicament stated “I love a challenge” and I love a mechanic who loves a challenge 🙂
So, I dropped the bike off, explained all the permutations I’d tried and left it with him.
He set to work trying some things I’d already tried along with some others I hadn’t considered.
The culmunation of his various tweaks means the Chopper is finally back on the road after 6 months and I can’t thank them enough.
Checkout their shop http://www.cyclecentreuk.co.uk/
Nigel's Adventure With His Purple Raleigh Chopper 
Never heard of such a problem. It would be very useful if you could explain the root of this unusual problem so the rest of us could avoid such an ordeal.
Aafter changing the chain the chain started jumping. So I bought new rear cogs (don’t forget I’ve converted mine to 6 speeed by adding another cog and derailleur). It just kept trying to jump from the small cog onto the bigger one and I tried all sots to fix it including numerous chains and cogs. Rob eventuaally waved his magic wand and it’s never missed a beat since.