2012 21.5" iMac video/audio file playback skips/stutters

I am running the base model with El Capitan. I am playing the files from the internal hard drive listening using the internal speakers. This occasionally occurs on using both Qucktime or VLC player. The playback skipping/stuttering occurs when audio levels are close to or at max level settings, and the audio levels peak on the source. When the same files originate from an external source drive, they play flawlessly. They also play flawlessly when I feed the audio output to an external amplified computer speaker set through the 3.5mm audio jack. Audio output level setting does not affect the playback stuttering using the external speaker playback. The video files are 480p MP4 and the audio are 64kb MP3. Talking to Applecare offers no solution.

It seems that the high audio peaks must be exceeding the current limit of a regulated power supply, thus causing the instabilities in playback?

Any help or suggestions appreciated.