programms halt computer

[sh0099]

i have the problem with ubuntu studio 8.04 with rt kernel and a 
limits.conf of

@audio - rtprio 99
@audio - memlock 750000
@audio - nice -10

with 1.5gb ram 1.4ghz pentium mobil

sometimes programs like ardour or on supercollider block the whole computer
so that i have any access any more and must reboot.

is there any configuration i can change to avoid this?

thanks
olaf