I'll try to get this Seafoam thingy but I doubt I can find it here in my country. Alternatively, I've enquired a local speedshop on carbon cleaning services. A whole job of injectors cleaning, motor flushing and carbon cleaning will cost me around USD$60.00 only. Will do it soon and let you all know if there's an improvement.

Thanks 98CoupeV6 for your advise.