hello ckmckee,
there can be lot of reasons behind this i.e:
*SCREENSAVER:
The most obvious reason why the laptop screen goes black can be a screensaver.
*BATTERY:
There might be some sort of problem in ur battery.
*HARDDISK:
As u have said that ur laptop is older so i think ur hard life span is expired.
*STANDBY MODE:
If a laptop is left idle for a long time, it sometimes slips into hibernate or standby mode automatically, in order to preserve power in the laptop battery.
*VIDEO/DISPLAY DRIVER:
Check out your video drivers.Maybe it is your video drivers that are malfunctioning.
These are some reasons.
Give me more information about it.