Hi Ralf,
I can only confirm that 3.10.9-rt5 gives plenty xruns and frequent Ardour crashes on my Arch install (x86_64, Asus UL30JT with i5 processor). I didn't get the other problems you mention though, but that's probably because I have a very light install which I only use for audio. Following a comment by the package maintainer on AUR, I downgraded yesterday to 3.8.11-rt8. The xruns disappeared and Ardour is usable again. It did freeze a couple of times for 2-3 secs, but no xruns and it recovered fine after that.
Thanks for sharing your experience. I'll check 3.8.13-rt14 to see if it still works for audio, and maybe solves the freeze problem too.