'96 Mustang 3.8 With A Constant P0306 Code

Ok folks I could really use your help here. I just purchased a '96 Mustang that was throwing a P0301, P0302 and a P0306 misfire codes. Well the car is bone stock and this is what I have done with it so far in regards to making her run smooth again. I have replaced the upper and lower intake gaskets, new coil, new wires, new spark plugs and new injectors. Now I am just getting a constant P0306 code for cylinder 6 misfire. Now it used to run smooth on a cold start and then like a switch flipped in about 5 minutes the idle will go down and the thing shakes like a bowl of jello in an earthquake. I attempted to change out the computer with a salvage yard one but it doesn't have the same number on it and it starts and runs for about 2 seconds then shuts itself off so I took that one back out. I have read ALOT of forums and everytime I find the same issue I am having, they NEVER post the results on how they got it fixed or even if they did or not!! So I am to my wits end and I am begging for all you gearheads out there to shine some light on this sucker!!
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I apologize but apparently I missed the line that said injectors....well then, have you done a smoke test or something of that matter to ensure that there aren't any vacuum leaks? I know you replaced a couple gaskets, but there's a lot of sources that can cause vacuum leaks...

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Double check your spark plug, wire and injector in that cylinder. It's entirely possible the wire, plug or injector is faulty. Try swapping parts around and see if the issue follows is. Swap the plug and injector to a different cylinder, swap the plug wire around. Double check your firing order is correct
I will let everyone know that it was NOT a spark plug, coil pack, wires, injectors, intake or a vacuum leak. It was exactly what I thought it was in the beginning but didn't want it to be. IT WAS THE VALVES IN CYLINDER 6!!! I had the heads redone and it runs like a new car. So this for anyone that is having a misfire code. Do simple things that cost close to nothing(spark plug) then borrow, buy or loan a compression tester and that will tell you ASAP if the problem is the cylinder itself or an electronic part.

****As for vacuum leaks.....that would not cause a specific cylinder to throw a code.