hi frank,
i really don't know if it will help you...
you need to be sure to use the last version of jack 0.100.0. also, i really recommand you to use the patched pam stuff to have realtime audio going on in linux. if you are using debian, you can follow the howto here it works on testing (etch):
http://www.ubuntustudio.com/wiki/index.php/Breezy:Rlimits-Aware_PAM
also, a good kernel with Ingo Molnar's realtime-preemption patch will help a lots.
http://www.ubuntustudio.com/wiki/index.php/Breezy:Vanilla_Kernel_With_Realti...
pat