Monitor loses Connection while playing Games, Pc works fine though?

Hey since a few days I have the problem that my Monitor loses connection to my PC up to 10 Minutes after I started a game, saying "No signal". My PC is still running, I can still hear the ingame-Sound from my Headset. This only happens when I play a Game, I can be on my PC for hours browsing the Internet (not playing a game) and nothing happens. Additionally, I tried playing the Games "Observation", "Surviving Mars", "Deep Rock Galactic" and "Fission Superstar X". For the first three, It happened after around 5-10 Mins. The last one I played for like 25 Minutes with nothing happening. The last game is a really simple 2D-Game, so maybe the Problem has something to do with the Workload that my Pc has when playing a Game? Im going to try playing Overwatch now and see if it happens there also, will update.

-Plugging the Cable from my Monitor to my Pc out and in doesnt fix it.
-I even switched the Cable with another one and it still happens, so It cant be the Cable.
-I also doubt that my Monitor is the problem.
-I already completely de- and reinstalled my Graphics Driver and that didnt help either.

I dont really know what I could try now. My Pc is a bit dusty, I am going to buy a can of air to dedust it. I hope that will work but Im not sure.

Do you have any other suggestions what It could be and how to fix it?

A bit of basic Info about my System:

Operating System
Windows 10 Home 64-bit
CPU
AMD Phenom II X4 840
Propus 45nm Technology
RAM
8,00GB Dual-Channel DDR3 @ 803MHz
Motherboard
ASUSTeK Computer INC. M4A77TD (AM3)
Graphics
ZOWIE RL LCD (1920x1080@60Hz)
2047MB NVIDIA GeForce GTX 660 (Gigabyte)
Storage
298GB Seagate ST3320418AS ATA Device (SATA )
298GB Seagate ST3320418AS ATA Device (SATA )
Optical Drives
HL-DT-ST DVDRAM GH22NS50 ATA Device

- - - Updated - - -

Update:
Same thing with Overwatch. I just wanted to test a variety of games to see that its not a game-specific issue or something.

If upgrading to the latest drivers directly from nVidia (not Windows Update) doesn't work, then it might be the result of excessive heat. You have to thoroughly remove dust from your PC, especially from the graphics card. You could use some air sprays or even better, an electric air blower. Also it would be a good idea to replace the thermal paste of the CPU and your graphics card. If unsure, ask a professional to do it for you to avoid any damage.

PS: Avoid the use of a hair dryer because if you accidentally set it to hot, dust will petrify and hard to remove! Even if you set it to cold it is not as powerful as an air spray or electric air blower. You also risk damage by static electricity. The same is true for vacuum cleaners, you risk damage by static electricity and they are not as effective as a strong blow from a spray or electric blower.