Audio Midi Setup "Source" is going crazy

Hello,

I couldnt find a fix for my problem so I had to post this.

Since yesterday my audio is behaving weird. When Im listening something, the audio starts, then it stops, then it stays for some seconds and stops again. I opened Audio Midi Setup, and the Built-in Ouput is selected and when I check "source", I can see that it changes automatically to "digital ouput", "headphones" and then again to "internal speakers" (when this one is selected, audio is fine).

Has somebody experienced the same problem? Ive got a Macbook Pro 15" 2009, 10.8.5