We have a 99 1500 ram van with a 3.9 it only misses at idle on number 6 cyl it has 139psi of compression ,a new injector, new plugs ,new wires, and it only misses at idle when you move off idle miss goes away i have a modis scanner and when you have it in miss fire detection test is when you can see it.map is reading 17 to 18 inches of vacuum at idle. also has injector pulse

 

Sounds like the ring(s) cracked.