codes P0500 & 0503 will not be associated with the misfires if you are still getting misfire codes. Thru the wheel speed sensors the CAB may transmit to the PCM/transmission info to limit speed. Both codes related to speed sensor malfunction/intermittent/erratic: clean the area on the wheel where the sensor picks up its signal, all the way around . You may have gotten grease on some of it, also clean the face of the sensor. Perhaps it is setting a little too far away, (the sensor sits under the wire guide) or the electric connection needs cleaning to make better good contact. NOT misfire related so long as those misfire codes continue to appear.
Last night I was reading the Daimler Chrysler student workbook on the NAG 1 ( read Crossfire transmission) " In vehicles equipped with the NAG1 transmission, the wheel speed sensors are used by the TCM ( transom. control module) to calculate transmission output shaft speed." I think this has to do with wheel slip in the winter driving mode. It is transmitted via the CAN BUS. However I also find this in a table " DTC p0501 Vehicle Speed Sensor Performance/ Rationality will set temporary LIMP mode and set the Malfunction Indicator Lamp." Perhaps your codes 0500 & 0503 do the same.
Seems like if changing half the plugs fixed half the problem you should change out the other 6 plugs; your making progress. Good luck.