Are you using your onboard soundcard (hw:0,0)?
If it's an intel hda card, you should also use 3 periods instead of 2. At least on all
my hda-intel systems, using 3 periods lets me go below 128 f/p, while using 2 would throw
a lot of xruns.
Also, as Jeremy pointed out, you should be rising your priority to at least 70 or more.
Good luck!
--
Federico Galland <federicogalland(a)gmail.com>