FPS drops after few minutes

[jerahya]

I have this strange bottlenecking issue with the build I made. The system is running fine and no overclocking. I tested it on 2 games so far GTA V and Dota 2. 2 CPU intensive games but i never had this issue on other builds in the past. Every time I run the game CPU and GPU usage is normal and FPS is normal. GTA V starts at 80-90fps 768p normal, GPU usage 90-100%, CPU is at 70-80%, after 5 minutes in the same scene fps starts to drop to 40-50fps, CPU usage goes up to 90-100% and GPU drops to 50-60% which is a clear bottlenecking, it does the same for dota 2. But the thing is my CPU temperature is only at 60+C just like when it is running the normal fps, barely hitting 70C, GPU is at 52C, I then test it on heaven and valley but everything stayed normal since it only stresses my GPU, run aida64 for couple of minutes to see if my CPU is thermal throttling but it is not, it stayed normal the whole time even above 70C. All cores running at 3.3-3.7Ghz. I even tried disabling hyper threading but still the same result. At first I though is something in the system that's hogging the CPU but task manager shows that only the game that is using the CPU. I then downgraded to windows 7 thinking its with the OS but still it does the same. Can anyone help me figure this out?

system specs is 
i5 2500k stock running at stock clock

Asrock H61M-HVGS
2X4GB G skill Aegis DDR3 1600mhz @1333mhz for 2ng gen.
500GB WD HDD
Gigabyte GTX 460
500watts PSU rapter

Can't be the GPU issue since I already tried GTS 250, GTX 560 and GTX 750 and it does the same.
I also build allot of system is much lower CPU specs but it never bottlenecks a on old card.
I already checked other similar issues but none of them solved my issue.

[Rolling Potatoe]

Is that a 1gb gtx 750? You might be running out of vram, even in 768p.

Other then that try monitoring your cpu and gpu clocks, usage and temps while you are gaming

when the fps drop happens, might be the different load causes something to throttle

the cpu. 

[jerahya]

it is a 768mb version but i it should only cause a bit of stutters.
I already run a test running gta v and hwmonitor but it doesn't show any abnormality on the clock, temo, vcore or what so even from the normal fps to the fps drop. it just decides to drop the fps and hogs the cpu usage for no reason.
I ran it on a fresh installed windows 7 only installed the driver vc redist and the game it self.

[jerahya]

Solved it I think. I decided to update the bios to see if it fix my problem and it did. the struggle was real though and the mini heart attack when I tried to flash the bios then it just shut down and power went out. Luckily the bios update never did run when the power went out. Updated to latest bios, had issue when booting since it doesn't seam to pass A2 to boot hdd in POST. Then have to search older update before the latest one, flash it again hopping the power would stay, had another mini heart attack when it doesn't want to boot or got stuck in splash again and shuts down. decided to clear cmos, fire the bad boy up, booted, run GTA V and it didn't drop the FPS for no reason like it used to. Still drops in CPU intensive area but not like before. Fired up dota 2 played couple of hours without fps dropping below 60 even though CPU is at max load.