Get the car tuned i.e. the whole works including air filter, plugs and fuel mixture.
If nothing improves then get a compression test done on the engine. If a cylinder is low on compression then it probably could be a bad hydraulic tappet. These tappet's health is very dependant on the type of lubricant you use and the frequency of its change. Dirty mobil oil is like poison for these tappets. These have small holes which can become clogged due to dirt in the oil.