Misfire moving around

Bought this van used, it had a rough idle but no trouble code. I had a mechanic at buying site change all the spark plugs, I asked for COP's too but he did not do it, but I just had to deal with it. After about eighty highway miles I finally got a check engine light resulting in a P0304. I changed all the COP's, reset the computer and it still had a misfire and threw a P0308. I changed the number eight COP for another and reset the computer. No change in misfire. At this point I thought it could be fuel delivery, so I put two bottles of Sea Foam in the tank. I drove the van for fifteen miles and pretty much had the same misfire that it has had all along. During this drive I got a P0300 first, then when I got close to home it threw a P0310. What would cause the codes to move around like this?
And for what it is worth, it runs really rough in reverse for whatever reason.