As I'm sure you've check by now (FAQ) Code 300 is a random Missfire. Also a flashing CEL is telling you your car is missfiring.
I would check all connections. Especially around the coil pack.
Check that black resistor that attaches to the coil pack to see if it's loose.
Hate to say it, but pull the plugs and look at them. Check the gaps.
What type of plugs did you put in?
What engine?
