Sudden fps drop after 10-15 min

I have an i7 4790k stock cooler, 8GB G.Skill Ripjaws X@1600, MSI Z97 Gaming 7 mobo, EVGA 750w PSU and a pitiful Radeon HD 5770 GPU running on Windows 8.1. I have used this rig for 6 months now and suddenly I lose half my framerate after 15 mins of gaming. This happens in all games. My rig is not dusty, my drivers are up to date and I have run Malwarebytes. My CPU temp is 60-65c in game and I have seen it crush video production at 80+c for 15 min with no problems and benchmarked with Intel XTU to the tune of 100+c before thermal throttling kicked in, so I feel like 60-65c temps are not an issue here. The FPS drop does not seem to be linked to GPU either. I can overclock my GPU a tiny bit with CCC and it seems to give me a few extra FPS when OC'd a few less with stock settings 50-60c in game. I get this huge FPS drop in say: Pillars of Eternity, while standing around doing literally nothing after 15 mins or so, a huge drop after 15 mins of doing all kinds of stuff and a huge drop 15 mins after alternating between waiting and doing stuff. Restarting the game does not help. I must restart my PC fully to get my 60+FPS back in any game, which holds solid for 15 mins or so until it goes back to 30FPS or less. Any ideas? I get my GPU is old but all my games run screaming fast before this situation has them running half as fast as they should and my temps arent out of control. I have a hard time thinking a GPU upgrade is the only fix. I am worried about the health of my PC.

Use MSI afterburner.. It will keep a log of temps and what not for your session and not just currently.. Because it sounds like something is throttling, be it your cpu or gpu... Make sure your fan is spinning on your video card, you said it was a older card so the fan might have failed.

Down load Burn test. You can use it free for a certain amount of time and it will tell you if you have any errors in, 2d, 3d, cpu, disk, memory and video play back. Person who tested my pc a couple of days ago did this to my pc and it came up as error for my 3d graphics. Change graphics and been fine all day. My graphic card started with low FPS then bad stutter and lastly I had crashes and blue screens.

http://www.passmark.com/products/bit.htm

@sSubZerOo: Fan works great. It is old so I can hear it, but works just fine. I do not have high temps. Just took apart and cleaned the GPU fresh thermal paste, cleaned fan and interior. Installing MSI Afterburner now.

I am getting 20 degrees difference in the MSI Afterburner reading and the CCC reading. MSI Afterburner reads 75c while CCC reads 55c in game. Which is correct? 75c does seem a bit toasty. I closed the game. I am now seeing 97% activity 425MHz GPU clock and 600MHz Memory clock, temp slowly rising from 50 up through 65c via CCC with PC at idle after closing games. Odd. I reset. After a reset CCC reads idle at 157MHz GPU clock and 300MHz Memory clock 38-41c 0% activity. So odd. Stock clocks for this card are usually like 850MHz GPU clock 1200MHz Memory clock. I have auto tuned with CCC to achieve 950MHz GPU clock 1350MHz Memory clock with fan at static 80 with no problems before now.

5770, right there, is it 2nd handed? refurbished or anything?

Well 55c seems too low while gaming, unless you were playing something not GPU intensive or you have some extreme cooling solution.

75c is more correct and its generally an accepted temperature for GPUs. Most of the GPUs have a temp limit of 95c or more...

Imagine that reference 290X is working continiously at 94c (well, we don't know how many GPUs will die after 2 years of working with these extreme temps)!!!

97% and 425MHz seems like you o/c the GPU and it couldn't handle the new clocks so it changin to 2D/Video playback mode.

Have you tried playing with default clocks?

Sounds like your GPU has faulty VRAM or its gotten a fault since you got it. The GPU isnt releasing the memory and after 15 mins its full, can you use the MSI afterburner log and when you get the FPS drop, post back the log at that time.

30 secs before and after it happens.