Perhaps an extension or incompatible third party plugin ...
From your Safari menu bar click Safari > Preferences then select the Extensions tab. Turn that off if there are any installed. Quit and relaunch Safari to test. If that helped, turn extensions back on then uninstall one a time to test.
If it's not an extensions issue, try troubleshooting third party plug-ins.
Back to Safari > Preferences. This time select the Security tab. Deselect: Allow all other plug-ins. Quit and relaunch Safari to test.
If that made a difference, instructions for troubleshooting plugins here.
Thanks but that wasn't the case.
I luckely solved the problem anyway.
The cause for this issue is the www.soundcloud.com site.
I always had this site open in one tab whenever the problem occured.
Close the tab and everything's back to normal.
Very strange behaviour.
Probably some HTML5 bug (new soundcloud is running purely on HTML5)