How is the idle? How many miles are on the car? It could be the coils, the injectors, it could be a clogged fuel system (filter, fuel rails, fuel pump), bad connection at the engine wiring harness, exhaust leak...you might just have to go by the process of elimination. Of course not replacing stuff at random. But you def wanna check and make sure everything is operating properly. Could take some time but you'll get it. Its probably something small.