p0300 can't fix

I'have 96 3.5 with 70,000 miles. I had intake manifold gasket replaced due to rough idel. After replacing gaskets engine light came on. code read p0300. I replaced plugs, wires, coil, air and fuel filter. Still reads p0300 and idels rough and became gas hog. I didn't have these problems before replacing gasket. Any suggestions?
Thanks

I found a site that explains OBDII codes, apparently they are not manufacturer specific, "p0300" states, "Random/Multiple Cylinder Misfire Detected" Could it be that maybe your firing order is not correct? Your coil pack should have the cylinder numbers marked next to each tower for the 3 coils, odd numbered cylinders (from the front 1,3,5) are on the passenger side and the even numbered cylinders (from the front 2,4,6) are on the drivers side, make sure you got the wires connected right.

Here is a link to the site I found explaining OBDII codes, OBDII Codes (http://www.troublecodes.net/OBD2/)