I think I isolated the problem. I'm using a software instrument (EXS24) in the song, and when I mute it, the computer does not stall anymore.
do a search on the forum, you will find this a common problem. it is a case of a corrupt plug-in channel.
if you have the perofrmance meter up and you notice one CPU spiking and the other hardly being used, then one of your plug-ins is "cycling" or feeding back. i think it is something to do with the programme not realizing that a plug-in is still engaged (the plug-ins are active even when the sequencer is not running) and not allocating tasks to the other cpu as it should.
you will also notice that even when idle the cpu meter (usually the left one) is hovering at around a third. the solution is to open audio config and option-click (bypass) each plug-in one by one until you notice the cpu drop significantly. that will be your offending plug-in.
reinstantiate the plug-in in another slot or on another track or audio instrument and it should be fine.
all advice you have received from justin is important stuff to know, but you should have been able to run 16 frozen tracks unless your buffer settings are really low. on a powerbook though, that's about as much as i would have thought you'd manage. he's dead right about an external hard drive if you're doing audio stuff on a laptop. don't forget the system needs your hard drive to do things as well.