Peugeot 307 car juddring when travelling in low gear 10-30

Hi, I have a question if someone could help I would be grateful. My car was recently serviced at 27,000 miles for a 30,000 service. No spark plugs were changed. After about 2 weeks. I noticed that my car judders when crusing from 1st to 3rd gear. The rev counter keeps jumping slightly. When trying to maintain a steady speed at say 10-15 MPS it keep up juddering and jerking. I have taken back to the garage and he doesn't know what it is. I took it to another mechanic and he plugged it into a computer but no faults were found. Had the RAC out and he says it needs to go to the main dealer for a full diagontic. I was wondering whether it could be sparkplugs that need changing perhaps something wrong with it or not or HT leads as my old car a Ford had a similar problem. Any suggestiosn would be appreciated. Thanks.