Vauhall Vectra 1.8 club ?

My car feels a little 'lumpy' at constant revs whilst driving. I had the spark plug change and the timing belt change recently. The car operated perfectly before that servicing. One other concern is my LPG system is cutting off and reverting to petrol.

Looking at the speedo cable - I notice it is retained on the one side with a clip but no clip on the other - so I can move the cable away from the bracket -- would this cause my problem.