Jerky at slow speeds

Thanks for the super detailed response Knight, I will definitely go through all the checks you've listed and get the bike running sweet before I fit a PCIII. I am commuting 30 miles (between Alloa and Glasgow in Scotland) twice a day mostly motorway but often with heavy traffic so speeds vary a lot - I did sail up to 120 one evening last week for a bit. So I'll probably stick to stock gearing. Thanks again for the advice.