Need help pin pointing DTC P0303 and sometimes P0300 as well

Hi,

I'm having trouble finding the source of why my friends car keeps throwing dtc p0303 and sometimes p0300 which I'll get to why I say sometimes on p0300. Well a friend of mine called me up last week asking if I could do him a favor of replacing 3 of his spark plugs. He said he replaced the3 in front but couldn't replace the 3 that are located under the intake manifold. I told him sure since I've done it to a few other Altima and Maxima's. Well when I was disassembling the intake I notice the intake gasket was sitting wrong not even linking up with the holes, valve cover gasket was leaking and looked and felt new and the PCV breather hose was broken on both ends yet the car ran perfectly fine. So I replaced the intake manifold gasket, PCV hose and correctly installed the valve cover gasket, and finally replaced spark plugs. So finally I assembled everything together And started up the car. When it started up It sounded fine but after 4 or 5 minutes after letting it idle it started to idle rough. First thing that came to mind was maybe I didn't fully push in ignition coil or harness all the way on one of the spark plugs So I checked all 3 while car was still idling nothing. I checked that I fully connected intake hose and check all surrounding vacuum and cooling lines as well as any electrical connections and everything was good. I checked for check engine light to be illuminated but wasn't so I sprayed water around intake and vacuum lines with spray bottle to see if maybe there was a leak and I know that method may not even if worked but tried it anyways, again nothing but shortly after shipping all that wen I was about to turn off the car the check engine light illuminated. Plugged in scanner tool and p0300 and p0303 came up. So first thing I did was disassembly intake to get access to cylinder 3 for p0303(cylinder 3 misfire) and swapped coils with cylinder 1 and 3. Assembled everything again and cleared codes. Started it up and again started perfect and after 5 minutes of idling the same symptoms happened again. Illuminating check engine light with this time only p0303. So I swapped coil#3 with coil 6. Again assemble it and same thing. So then I decided to install the old spark plug and remove the new one and check the gap on both old and new new one was good old one wasn't off by much. Again same symptoms only this time it through out p0300 and p0303 and I forgot to mention the code relating to cam position sensor bank 1. So I replaced bank 1 and 2 cam sensors and checked the wires and harnesses to all 6 coils and cam sensor and everything looked okay. Once again same symptoms only this time dtc was p0303. So the only thing that I can think of is maybe injector on #3 is starting to go out or the o ring is bad on it. So I was wondering if I can get some advice on what I may have done wrong or how to find the problem. I feel bad that his car is now having these problems after all I did was replaced spark plugs and a few other things. So any help or advice will be greatly appreciated. Also thank you in advance for taking time to help me out with anything.

Thank you,
Rob