It sounds like a different issue, so I’ve moved it to a new thread.
EDIT:
Could you please give some more information.
What are your settings:
System Settings → Energy Saving
Specifically:
- Suspend Session → After a period of inactivity
- Display and Brightness
Does it wake up ok if the monitor has only just turned off?
If so, roughly how long does it take to become problematic?
Could it be trying to sleep but not succeeding? If so, have you tried preserving video memory (as in the other thread)?
Also some system info may help.