Interesting problem, when I start my car, it idles at about 2k for about 15 seconds, then the idle slowly drops, and gets really rough, until it finally settles around a bouncy 350-400 RPM. The CEL was on, so I checked codes and came up with 21 and 52. Now, I didn't think an o2 sensor problem (21) would cause problems at idle because I thought the ECU didn't look at the o2 sensor until around 3k RPM. The knock sensor (52) however seems to me the more likely cause since the engine does behave like timing is being retarded when it starts to die off. Any thoughts? TIA.