Everything was working fine during rehearsals. But when we got to tech/dress^ it started crashing.
One of the biggest factors in computer crashes is power fluctuations. Here are some rules to follow for setting up a computer in a theater:
Computers should always be on dedicated power. This means that you should not have the computer connected to the same outlet as any other pit or theater devices, including stand lights.
If the computer is plugged into a power strip, do not have any other devices plugged into the same power strip.
NEVER plug the computer into the same circuit as any lighting or other devices controlled by a dimmer. If you do, you can destroy the computer equipment.
Dedicated power means more than just an outlet in the wall with nothing else plugged into it. That wall outlet may be part of the same circuit as your lighting (or other) devices, in which case you will not have remedied the above problem.
When in doubt, speak to your theater's electricians before plugging things in.