|Monitor the temperatures in real time by having HWMonitor display in the tray next to the clock. See what the temperature is when the screen goes dark.|
Laptop processors are set up to run at variable speeds depending on the demand. So if you are hitting 69C the processor is probably slowing down to maintain that temperature.
You may need to partially dismantle the case to clean things better.
Get you see a lot of dust come out when you blew it out?
Your temperatures are kind of high. My desktop CPU is currently running 31C. That is about 10 degrees F. hotter than the room temperature.