diagnostic tools to trace the reason xruns are happening?

[Ralf Mardorf]

Is CPU frequency scaling set to 'performance'? If you start a GNOME
session, I bet it will switch the governor to 'ondemand', if so, you
only need to set CPU frequency scaling to 'performance'.
Using a PREMPT kernel have proved a receipt for issues when doing audio
work. You need a  kernel PREEMPT RT or kernel ex 2.6.39.