I've never had a problem with
QLab like you've described. I do always have the "force core audio to stay awake" checkbox checked, and in system preferences, I have it set so the screensaver never comes on, the display stays awake, and the hard disk never goes to sleep. So far, I've not run into a problem with these settings on a 2GHz Core Duo MacBook Pro. I bought it two years ago. Though I'm about to add an M-Audio Firewire audio/
MIDI interface, so we'll see how it behaves then.
I know a local outdoor venue here uses QLab for their SFX. They have a Macbox (as my Apple-employed friends call it) backstage in an air conditioned room, running QLab with a MOTU Traveler interface. Actually, they have two of each for redundancy, and a remote keyboard/monitor/mouse at
FOH. The computers and MOTUs used to live out at FOH, but the heat would cause the MOTUs to lose the audio sync and then drop the audio. Cues are fired by a MIDI command from the Cadac console, and to the best of my knowledge, they haven't had a failure in the two years they've been using the current setup.
Herr_Sprecker, what's your setup for QLab? Perhaps it could be an issue like I mentioned with the interface losing sync.