can anyone please help me problems with 2000 expedition xlt

hi, i have a 2000 ford expedition xlt 4.6L with 110,000 miles on it. i was driving it one day and it started to loose speed and something popped and my check engine soon light came on. i got it home and i checked the codes that came on it first it said p0303 and p0308 so i looked it up and it was misfiring codes so i checked the spark plugs and sure enough the number 8 plug was broken in half so i changed all the plugs in the truck. I had bosh plugs in it when the misfire happened so i changed them all back to motorcraft. figured changing them will fix the problem that wasn't the case so then i decided to change all the coils and the boots that didn't fix the problem so i changed the fuel injectors and still the problem is still there. when i finally get the light off i took it for a test drive the truck run great but when i go up the hill the check engine soon light starts flashing but when i go back down the hill it goes off. I also changed the fuel filter i took it to a machanic and he said it was the piston rings but i don't think thats what it is. the reason why i say this is cause i asked him how sure is he that thats the problem he couldn't give me a sure anwser. can anyone please help me my truck has been sitting for three months now and i feel like i'm going out my mind. i also let a friend look at my truck he said it sounds like a sensor problem. let me add this a few days before my truck had this problem my lights wouldn't go off so i checked the wire harness under the inside door panel and there was a few wires burned up so we fixed that and right after fixing that i drove it and then the misfiring started. could this have triggered any of these problems i am having right now. thanks

Put it on a battery charger for a day and then pull all of the spark plugs and do a compression test, that will tell you if there is a problem with the piston rings or not.

bad coil packs maybe? Ford's are know for them going bad and having misfire problems.