Xruns during playback with Ardour 2.1 / Jack

[Robin]

Hi everyone,
I'm trying to run Ardour (2.1) and Jack together and I am having trouble
with xruns. Once in a while ( I can't see any logical occurrence )
Ardour triggers loads of xruns when it reaches the of a region on a
track during playback.
One bodge I have come up with is to paste empty regions so as to fill
the gaps between regions on the "problem" track this works ok but is
annoying to have to  do.

Here is my config:

Sound card 

Alesis io2 (usb)



Jack 

realtime:yes 
priority:89
Frames / Period:512
Sample rate:4800
periods/buffer:3
Port Maximum:1024
Time out:1000


If anyone can help me I would be very grateful.
Regards,
Robin