Lights dim at 5K rpm

Hello, I have the same problem as a lister in an earlier post. My lights dim when the RPM's get above 5K rpm. The previous post suggested that it could be a
bad coil. Anyone else have this problem? In the previous post the person said he would give it a try but never posted if it fixed the problem. I have no caution lights on when this happens. Any info would be welcome...
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