Black Screen/Can't find solution

Hi! I’ve had an issue for awhile now that is pretty aggravating. This was not an issue on my old PC, which was considerably worse than my current PC, and I’ve searched everywhere for a fix and can not find one. After about 15-75 minutes of being on a server, my game will lag, both my monitors will go black, then come back up, but Gmod will still be black. Sound is occurring and I can even still interact in game, but the screen is black no amount of alt+tabbing fixes it. I’ve verified the cache on everything source, even things I don’t have mounted (just for fun), reinstalled Gmod, ensured my monitor drivers/gpu drivers/cpu drivers/game/steam/everything is up to date. It still occurs, at any point between 15 minutes and a little over an hour. Here are my PC specs:

Processor: Intel® Core™ i5-4690K CPU @ 3.50GHz
Manufacturer: Intel
Speed: 3.5 GHz
Number of Cores: 4
Video Card: NVIDIA GeForce GTX 980
Manufacturer: NVIDIA
Chipset: GeForce GTX 980
Dedicated Memory: 4.1 GB
Total Memory: 4.0 GB
Memory: 16 GB
Operating System: Microsoft Windows 7 Professional Edition Service Pack 1 (build 7601), 64-bit
Service Pack: 1
Siz:e 64 Bit
Edition: Professional
Display Maximum Resolution: 1920 x 1080
DVD: ASUS DRW-24B1ST j SCSI CdRom Device
CD: ASUS DRW-24B1ST j SCSI CdRom Device
Drive: ASUS something or other
Size: 931.4 GB
Free: 97.3 GB

Can anyone help me solve this? I just can’t find a solution that works for me. Thanks!