1997 Ford F-250 Misfire #8

About a month ago I was driving and the engine started missing on a cylinder. I drove down to the auto store and checked it on their reader. It showed a misfire on #8 cylinder. However, the next day it ran perfect and I left everything alone.

This just happened again the other day and this time the misfire will not quit. I went down and bought a new coil for #8 cylinder and replaced this with no luck. I took this coil back thinking it was a bad coil and replaced it again. The truck ran fine for about 2 miles and dropped #8 cylinder again.

I did not replace the one spark plug thinking this might cause a check engine light with only 1 new plug. The plugs have about 40-50,000 miles on them.

This same problem happened about 4 months ago and I replaced the coil on #7 and everything worked fine. However, this is not working this time.

Any ideas as I need help.

Regards, Brad