Have you tried replacing the cap and rotor again? If it was a cheap one odds are it was an aluminum POS, which don't take much to ruin, I toasted one on my 1990 Dodge that I used to have. The fix was replacing the Rotor and Cap again with good grade copper ones. I run autolite single platinums in my Dodge which is also a 1992 W-250 with the 5.9L V8 TBI.