I looked at my service manual re: your misfire issue, and there is a 'misfire test' you can request to the ECU, I will assume it fails this test, hence giving you the p0300. Im not sure what your obd setup allows you to do. Also going off an older post it was suggested to do the idle relearn, see here: http://www.aveoforum.com/forum/f108/...98/index2.html

Good to try the cheap stuff before driving it off a cliff, or so I learned LOL