Ive bought a set of plugs today hopefully to get over the problem although Im not sure there the right ones as I have dual fuel, the guy said regardless of dual fuel or not these should be fine NGKBPR6EFS-15. Reason im doubtful is that the LPG installer said you need special plugs for dual fuel??

Anyway I will fit tomorrow and hopefully it fixes the problem. Its strange because when I run on petrol its fine until I hit 80km's then its lumpy, I can accelarate through this and once im 90-100 its okay again! any advice will be gratefully received thanks