P0300 code. Car is running really rough one time and running fine next time.

p0300 is a misfire detected code. A misfire can occur during a number of different conditions. A coil pack, failed plug, plug wires, something as simple as a leak in the intake, or a fouled fuel injector could cause a misfire. Plug wires should be changed at the same time as the spark plugs, and these days I recommend changing every 30-35k miles due to ethanol gas causing so many issues.