Engine is missing

Within the last 1000 miles, I had a new O2 sensor, gaskets replaced on the intake manifold to pass smog. The car was clean, but the "check engine" light would stay on.
The other day, When I started the car, the light came on and there was a noticible missing, shudder.
The OBD indicated PO 300, 304 & 308. This indicates that cylinders 4 & 8 are misfiring. I changed the plugs, but did not help. Is there a way to troubleshoot this problem, perhaps checking if coils are good, etc? what else?
Please help.


Chuck