Have either of you checked the temperatures of the machines? Maybe a batch had the heatsink poorly attached or something.
First thing I thought of. Used the shareware app Temperature Monitor - values were quite reasonable compared to my G4. Issue ONLY occurs in iMovie - nothing else. I'm of the opinion that's it's iMovie mucking with the power-control circuitry. Apple phone support has run through every single test possible. Client is furious since numerous errors also occurred on the order and erroneous info was given about compatibility of early-model 23" studio display, neccessitating adapter (at further cost). Suffice to say, the 'glow' is long gone.
Marketing really must stop forcing ship dates that don't allow for proper testing of the products.