It may very well be out of spec caps in the power supply. From my experience though, a certain amount of this is normal. In transient passages, at high volumes, the amp is requiring more power, and the lights suffer. I noticed this the most with the pioneers I've used. I've had a CR1020 CR2040 CR640, and still have a CA2010 and CR2020 all of which have some dimming at high volumes even after recapping. I'm sure someone will chime in with a little more of a technical reason if thats what your after.