Games slowing to a crawl after ~15 minutes of gametime

[Agent531_C]

Recently, I have been playing a lot of overwatch on my gaming laptop, and it has run flawlessly up until about a week ago. It would run great on high settings before, but now it will slow down to about 5 fps after 10-15 minutes of playtime. It also seems to do this on other 3d games, specifically subnautica, tabletop simulator, the witness, and the witcher 3. I hardly ever hear the fan running loud while playing intensive games, so I believe it to be a problem with overheating, but that doesn't answer what is going on with the charging.

One other symptom has been what seems to be the power cord disconnecting in games. In my brightness settings, the max brightness while plugged in is 100, but while unplugged is only 80. While playing games -and not once outside of games- the brightness fluctuates between 80 and 100 constantly, also causing slowdown. I'm positive it is not the charger which is causing this problem, because I have a second one that has the same issue.

My laptop (Gigabyte p55) has an i7-5700hq, 8 gigs of ram, a geforce 970m, and is running on windows 10. I haven't had any problems of the sort in the 7 months I have had the computer.
I suspect it to be an issue with overheating, but with it coming on so suddenly, I am unsure as to what exactly would be causing it. If it is overheating, would an accessory such as this be helpful?

Edit: Now after exiting a game after it has slowed down, the entirety of my laptop runs slow as well. the browser can barely function, youtube videos buffere every two seconds, and it is in a hardly usable state being this slow

 

[Mr Hat]

This really sounds like an overheating issue.
Just start off with cleaning your laptop out. Just open it up (some screws on the bottom), and take a vacuum cleaner/compressed air and blow/suck it out. Watch out for those hidden spots around the fan, there might be dust hidden behind the plastic. Don't give up quick, keep going until you're sure there isn't any dust left. Try this, and tell me what happend, if this didn't fix it, I've got some other ideas too.

And, also, I think there might be a problem with your charger, not giving power all the time, switching on and off. These aren't too pricy, and you can buy them everywhere so I don't think that should be an issue. Don't buy one yet, let's fix the other issue first