This may or not help but thought I would throw it out here. I have been having same code P0300 and my cylinder misfire was #4 with my Actron 9145. I did everything to find problem. I found one problem was O2 sensor wire was touching a metal bracket and giving false readings. Then my muffler blew a hole due to old age and misfiring helped it along. Replaced muffler and a bit better but O2 acting strange. O2 only a year old and about 8K miles. Cat is self destructing. I believe the failing cat killed the O2. New Magnaflow Cat and Delco O2 sensor is on it'* way. Check these out. Good luck, I feel your pain as I have been fighting this for about 3 months now.