U just need to Re-adjust the CNG Kit and if it still does'nt work than may bee clean the throttle body/ Plugs thats it. For Engine check light, Check all the sensors, clean the MAF.
If the same problem persists while on high speeds than may be its the advancer, Both just cant Malfunction at the same time.
Do note that the Vitz is a mighty machine (with all the goodies) considering local made toys available in pakistan. respect it and it will serve you, like all cars........:d