Easy:

Somewhere in one of your intake hoses, there is a probably a tear or a hole that only gives you problems when you really go into boost. That's when the computer give the engine gas for the mass of air it expects, but instead, it only gets a fraction of the air because it's dumping the rest of it out of the ripped/holed hose.

That equals a very rich mixture.

The engine could be running fine the rest of the time, but only running rich for small periods of time. therefore the plugs might not look like they are running in a rich environment.

It would be the most likely cause. Some holes are VERY hard to find, but if you're careful, you'll find it. And it takes only a small hole or tear to cause that problem.

There are other potential causes, but this is by far the most likely.
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chris herbst, near chicago