Best case scenario is that it is a bad injector. You could try swapping the bad injector with one of the others and see if the problem persists. If one of the other injectors works fine in cylinder 2 you just need a new injector. Worst case scenario is a problem with the ECU. Unfortunately I have heard of several ECUs failing and causing this problem. Only fix is a new, and expensive ECU. One other thing I would try before buying a new ECU is to check all the connections at the ECU and injectors, and clean the contacts, apply dielectric grease, and reconnect.