Knocking while Idling, and sluggish at slow speeds.

While idling my i30 knocks, I don't know cars very well so sorry if my explanation is hard to understand. When Idling it knocks real hard, and the RPM meter goes down with each knock. At fast speeds it is fine. I already had igniton coils replaced, and spark plugs. Then got a code to replace the O2 sensor (upstream), replaced that. Then my car stalled on me, and wouldnt start, check engine light was on. Forgot to clear the codes. Did that, then got another code for a coolant temp sensor. Replaced that, and the knocking actually stopped for 2 days. Now its BACK. Can anyone shed any light on what the hell is wrong with it? The knock sensor? the fuel filter, idle control valve?! I'm going crazy, and keep getting different codes and the problem is STILL there. Please help!