[fastfredracing]

Most likely coil packs, or plug boots. Would be nice if you could get it to missfire enough to set a specific cylinder missfire, and then you could nail it down better. I'd lightly mist the coil area with water, and drive it hard under load to try to make it missfire and set a hard code for a specific cylinder .
Also, check around the maf sensor, and air intake boots for anything loose, cracked, missing etc... You could spray a little ether to help pinpoint a vacum leak.
When you changed the plugs, was there any signs of spark leakage to ground? Usually a whitish looking powder right at the bottom of the plug/coil boot. Did you grease up the boots with dielectric grease when you put them back on? All this stuff can help to cause intermittent missfires under different humidity and load concitions...
Good luck!