laptop using external monitor, Screen black out after 15 mins gaming

Hi,

My asus laptop has been doing fine with gaming with no issue.

I want to connect the laptop to a monitor for gaming. After 15 mins of gaming, the external monitor goes black (as if in sleep mode).
I open my laptop lid and the screen goes black too. The laptop is still running because I can see the laptop is still online in the online game from my dekstop PC.

I can play the game without using the external monitor,everything is up and running with no problem.
When I use the external monitor for browsing and download,the black out screen did not happen. There is no problem using the laptop with external monitor for other purpose.

It seems the blackout happen only when I gaming using the laptop with external monitor.
I have set no sleep,no hibernation,no shut off hardisk in the setting but could not solve the problem.

Anyone know what might cause the problem??

Thank you.