If your Ford Crown Victoria does not have any engine symptoms the simplest thing to do is to reset the check engine light and drive your Crown Vic to see if the light turns on again (and P0305 appears). If trouble code P0305 does not re-appear, go ahead and drive for 200-300 miles (over the course of a week) and get a smog check retest.
If the check engine light does turn on again after the reset then we have a problem. You want to first inspect the ignition system. Try swapping the number 5 cylinder coil-on-plug (COP) with one from another cylinder to see if the problem now switches over to the cylinder on which you placed the number 5 COP. If you now notice the misfire has moved, we've isolated the problem to the COP and replacing the unit should fix your problem.
If it's found that the COP is fine and the problem is something else, we'd recommend checking cracks or hairline fractures on the intake manifold near cylinder 5. A crack will cause an intake vacuum leak and might result in a cylinder misfire. You should also conduct a engine compression test to make sure all cylinders have equal and adequate cylinder compression.
