Random misfire - engine misfiring?

Engine Performance problem
2000 Nissan Frontier 6 cyl Four Wheel Drive 169000 miles

This problem just star 3 days ago : first going good and I fill like a missing just for second and then run normal, second day was worst and I changed the cap, rotor, spark plug wires, and spark plugs has 7k miles on it, same is fuel filter and the most important thing there is NOT check engine light on, well the third day I started when its cold the idle is ok, but when rich the normal temp. Star the problem and I see some thing the right side of the engine (Catalytic ) star smell like overheat metal help please