Help me diagnose an intermittent missfire?

My 1996 Bravada has a missfire, it runs fine when cold, the miss starts when the engine is fully warmed up--but not every time. Once the engine is warm, the miss comes and goes as though by the flip of a switch. It feels like a regular, recurring miss on one cylinder, sometimes two. Sometimes the miss will go away when I blip the throttle, sometimes it gets worse when I do that.

Scanned codes are rear heated O2 sensor inactivity (the one after the cat) and Intermittent Missfire. I replaced the O2 sensor, cleared codes and the code came right back. Backprobe of the O2 sensor harness shows voltage in proper range, and definitely not inactive. The sensor wire swept between .2 and .9 volts 6 times in 20 seconds, which (as far as I understand) is what it's supposed to do.

Trying to chase down the miss, I've replaced the spark plugs, plug wires, rotor (cap is 1 year old and appears to be new) and coil. Also I used a soda-can to block off EGR, none of these made any difference. Any idea what else to test?