Ok, I just completed another repair to my '99 F-550. It started running strangely a few days ago and checked things out and found that the glow plug wire for #6 was shorting out against the push rod!

I took the valve cover off to also discover this: The plug was on crooked!

I removed the right side valve cover to see that the same problem was going to happen there and replaced that side too. See the wire rubbing on the push rod.

The new wiring harnes is routed a little different to make sure that doesn't happen again. Plus I bent them as far away from any moving object as possible.

It wouldn't surprise me if all the '99s with certain production run have this problem.