rough idle

Hello all:
My son has a 1996 Crown Vic police car. It has been a great first car,but is now having a rough idle problem. The OBD code suggest a P0303, which is a cylinder 3 misfire.
We took it to two different mechanics, and paid a total of $250.00 in diagnostic fees. Confirmed not a bad valve. First guy said bad plugs. We changed them. Second guy said bad plug wires. Put in new plug wires, no change. We asked at a parts place. They suggested new coil packs. Changed them, still no change in the rough idle.
We are now relieved of $450.00, and still have the problem. I am thinking of a bad fuel injector on number 3 cylinder? Possible? Parts guy said injectors are a very rare problem on a Crown Vic.
It idles fine in neutral or park, but the minute you put it in drive or reverse, it rocks like a boat on the high seas.
Thanks to anyone with ideas!
Roger.

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i would check the compession before going any further

Bad injectors are rare, but dirty ones are not. If the compression test recommended by lotec is good. Go to a mechanic get a fuel injector cleaning. No tellin' how many hours that thing spent idling behind donut shops. May just be all carboned up.