if your motor is a JDM motor, which I'm sure it is.. this problem is caused by the fact that the motor is designed to run off 100 octane fuel ( because they have that in Japan ). what grade fuel do you use? even with 93 you'll get this surging every now and then. the solution is to install a fuel press. reg and just raise the fuel pressure about 2-3 pounds and it should run better. this is one theory, the problem could allways be something else but this is one of the common ones.