After 150,000 on the original factory plugs I replaced them all with Bosch Platinum, single tip, plugs. After a couple of months my car began to idle roughly at stops but not always. Then one day it got so bad the check engine light went on and stayed on. The problem was with the #6 cylinder. Upon removing the plug I noticed the tip was gone. Replaced it with same type and about a month later same thing. This time I used the type with four tips. Another month or so, #1 cylinder, remove plug, no tip. Few months later a loud backfire and a lot of power lost. #3 tip is gone and #4 but #4 was more severe because the coil sleeve is torn up and separated from coil. I was able to remove tipless plug but the treads were damaged and could not properly install new plug. Anyone have this kind of experience. Car is 1999 Chrysler 300M, 170,000 miles. So far the four prong plugs are all OK. Called Chrysler HQ, local Chrysler dealer, Bosch and other mechanics, no one knows what could have caused this.
The Bosch book "Gasoline Engine Management" has good information on troubleshooting spark plugs by appearance. It's a very useful book all-around.
They distinguish between damage to the center electrode and damage to the ground electrode (or electrodes in the case of the Platinum+4's).
Center electrodes can be melted or eroded by thermal overload due to auto-ignition (i.e. knocking). If the auto-ignition/thermal overload is bad enough, you can end up with simultaneous damage to the center electrode and ground electrode(s), and sometimes you end up with soft/spongy/cracked insulator tips (the white part).
If it's just the ground electrode that is eroded, for causes they point to:
It sounds like you have foreign objects coming into the combustion chamber. Barring the plugs themselves from being the problem (incorrect heat range shouldn't cause this, wrong plug length could), are there any holes along the intake tract where foreign objects could be ingested by the engine?