It'll most likely be the coilpacks, if the stock ones are still in there. Extremely common prob.
It will seem fine for a couple of weeks, then you'll notice a misfire and it'll get worse from there.
Get yourself a consult cable from here and you can confirm what the fault is. Fault code 21 = will be coilpacks. If you took it a dealership or mechanic, this is what they'd basically do - plug it in and see what fault code is on the computer.
There is another way to check by bridging the pins on the consult port (just search on here).
I got JJR ones for $399 delivered, and put new plugs in as well.