Slight Misfire HELP!!

[95talonracer]

I just rebuilt the entire car and it fired right up and ran good for a few days and then a valve spring broke. I fixed the valve spring and it ran just as good as new. Then when I was driving it home one day it started making a wierd running feeling just like it did when the valvespring broke but not even half as bad. There is a misfire and it threw the code 0300. It isnt as bad as when the spring broke on idle and you really cant tell something is messed up until you are under load but at cruse it is fine. here is what I checked

I put new plugs in twice, New plug wires, swapped coils, transistors, ecu, and am stuck, can a 91 CAS be the broblem or no?