I have a feeling its a vibration issue that's distorting the sound (especially prominent with V8 HEMI's). The good news is there are several great isolation devices that could help mitigate the vibration effect and produce a better quality sound. I would recommend that you rest your Wife's radio on a Sistrum SP1 Isolation stand. You may need to re-configure your entire vehicle to accommodate the device which in turn may upset your wife ...but the hell its all about the music right? I also agree with the other member suggestions as well (clever lock, footers, etc...) the culmination of all these devices may take you where you want to go.
Best of luck!
Best of luck!