Even that machine will slow under load. However while it was nice for you to post temperatures it's often a sign that you will have to write a lot to get the PC builder (you?) to just do the work of redoing CPU/GPU/all heatsinks then go over what could be software or driver issues.
The dropout is my clue it's time to do all the usual work and not waste time on software yet. Here's a nice article on another GPU. Nice pictures on what to do. http://www.tomshardware.com/reviews/radeon-r9-290x-thermal-paste-efficiency,3678.html
Bob
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Operating System
Windows 7 Professional 64-bit SP1
CPU
Intel Core i5 4670K @ 3.40GHz 33 °C
Haswell 22nm Technology
RAM
8.00GB Dual-Channel DDR3 @ 666MHz (9-9-9-24)
Motherboard
MSI Z87-G45 GAMING (MS-7821) (SOCKET 0) 29°C
Graphics
SMS23A550H (1920x1080@60Hz)
Cintiq 24HD (1920x1200@59Hz)
2047MB NVIDIA GeForce GTX 750 Ti (ASUStek Computer Inc) 34 °C
Storage
931GB Western Digital WDC WD1002FAEX-00Z3A0 ATA Device (SATA) 35 °C
Optical Drives
No optical disk drives detected
Audio
Realtek High Definition Audio
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So I've had this problem for nearly 8 months with no solutions I've found anywhere working. While using my computer, sometimes randomly and others seemingly just when under a lot of workload (playing Final Fantasy 14 or using my Cintiq 24HD)
It will suddenly drop in performance. Sometimes, but not always, it will begin with the screen going black, and the monitor searching for input. Upon returning to a display, often Chrome will be a window with just blank unclickable screen inside the borders. Over time it will refresh, or black out again and come back useable.
If it doesn't start this way, while in game or out, It will begin with the visuals and audio kind of "clipping", there is no proper descriptor I've found. The FPS basically drops from 70 to 15-20, and the sound is basically coming in at that rate as well. It eventually devolves to the point where I have to exit or restart my computer. Sometimes it affects computer-wide performance, sometimes just exiting the game fixes it.
I know my computer is able to play it on average settings, because sometimes it will play just fine. But the issue returns day and day again and I haven't been able to figure it out. Thank you for your time!

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic