Misfire and Spark Plug Problem

[petstang]

I have a 99 Sienna with 183K on it. Spark plug wires have ~30K on them. Sparkplugs have 30k. Injectors are original.I had a misfire in cyl 4. I removed the plug and saw the insulator was cracked by the electrode. I replaced it with a NGK plug. Problem went away. Then soon after I had a misfire in cyl 3. The plug looked OK, but I replaced it with a new NGK and problem went away.Now about 5K since the last NGK plug replacement I got a cyl 4 misfire. I moved the coils around but the problem never moved. Plug looked OK, but I replaced it anyway with a new NGK plug and problem went away.Does anyone know whats going on here and why I can temporarily cure a misfire by replacing plugs that should still be good?Thanks for your help,John

[wiswind]

Make sure that your are using the correct plug....the number should be on the emissions sticker under the hood and in the owner's manual.
My owner's manual (for my 2003) lists a DENSO number and a NGK number.....either is fine.....although I would want all NGK or all DENSO.
NOTE that they use different plug numbers between different years of 1st Gen Sienna.

The 4mm "Power Plus" plugs are rated for ONLY 30K miles whereas the OEM specified 7mm plugs are good for 100K miles.