8th cylder is misfiren

What is your question?

i have a 2000 navigator and check engine light came on so got it checked end up being code p0308 8th cyclder misfiren so brought it to the shop had the coil plus plug changed drove 20 mins up street light came on again brought it back guy changed it just to make sure it wasnt something he did this time two days later light came back on put it back on machine still reading nunber 8 misfiren what do i do