Well, I got the Trinity installed tonight and went for a drive. The stumble I have coincides directly with knock/timing retard. I got up to -9 deg of knock retard and it would go to about -1.5 and up to about 3 at light-medium load, i.e. going up a small hill without downshifting. I did have the 87 performance tune loaded, but the feeling was exactly the same as without the tune installed. Here are a couple of photos of the plugs. Doesn't show up as good as in person, but you can see the ones that are white are out of cylinders 4 & 5. Anyone have any other ideas?