could be your fuel...might require higher octane rating. or, could be where you fill-up din, baka binabaha and may halo tubig fuel nila.
had the same problem before...i was using shell unleaded, then i shifted to premium, then tried velocity. as advised by a fellow forumer, change brands or change stations. i changed both brand and station....solved! try mo muna ito then if it still persists, do the next step; tune up, timing, check distributor, plugs...