Xruns on Arodur but not in qtractor

I have some (every 2 seconds more or less) Xrun. But it does not occur if I have ardour switched off (system -> system) or if I record using qtractor.
I have tried wit an usb generic sound interface and using the one integrated on board.

As the jack configuration is the same, and as well the audio interface, and the only difference is qtractor / ardour, I suppose there is a problem in some part of ardour configuration. Am I right?

May be the solution is recording on qtractor, and copy everything to ardour?

Sorry if this topic is in another post, but I could not find it.

I forgot,

versions:
ardour: 4.0.111+r12836.21
in ubuntu 14.10

I think the configuration of jack is not necessary, as it works fine on qtractor.