Misfires??? Please help me figure this one out!!

Hello. I will try to keep this short, try! I have a 2000 Ford f150 4x4 with the 5.4 Triton. This past winter I had a misfire (P0356) on # 6. As it was cold as hell I just changed the coil. Ran ok for a while then I got it again along with # 8. #8 must have been short lived (intermittent)because I replaced #6, cleared the codes and it ran well for a month! It started running scrappy again so I bought news plugs, injectors and coils for a complete tune up! I replaced the drivers side(plugs,injectors and coils) with all new parts. Haven't done the passenger side yet. I wanted to see if this took care of my problem!
I started the truck and it ran smoothly. I hopped in and started driving down the road. Ran great for a quarter mile or so. Then, all of a sudden it starts missing bad!
Read the codes and P0355 thru P0358 pop up??
If I turn it off and restart it, it may run smooth and beautiful for a mile or so but then out of the blue it cuts out and the four codes re-appear!!?? I have no clue what to even look for??
Catalytic converter plugged?
Please someone help me figure this out!!
Thank you.........