78 is kinda hitting that critical mark, albeit at the low end. There may be potential that over an extended period of high usage, the GPU is hitting critical temps and is causing the BSOD, which could explain why your system would have POST beep errors after a BSOD as it's still not properly cooled down yet. You may wanna check with HWInfo
the temps for your other hardware as well. Better yet, log it for us. When you start it up, select "Sensors only" and then start a log of it (default settings). Play a game until you BSOD, then send the log to us. Getting us two logs of two separate BSOD incidents would help even more as we can compare.
As for the beep code itself, I think I misinterpreted it. You previously said it was 6 beeps, yet you mentioned 5 long beeps means no VGA as mentioned on the manual. I take it the manual does not mention the 6 beeps (short beeps I presume?) you're hearing, yes?
Regardless, you are experiencing intermittent beep codes. Therefore you have
a hardware problem, there's no other way to explain it. Again, make sure to re-seat the video card and check the slot and card connector and clean them if they're dirty. As an extra measure, make sure that if there's an aux power cable connected to the video card that it's properly connected, and you may even wanna go ahead and reconnect all your other cables (drive power/data cables, PSU cables, etc.) as a just-in-case.
Outside of that, it's really hard to determine this. First thing I'd recommend, if your motherboard has onboard video, you may wanna switch to that temporarily and play games (if you can) to see if it BSODs still. If it doesn't, your video card is most to blame. As for the hardware tests themselves, you said you already completed them. For Prime95, how long did you run it, and on what settings? If Large FFTs, run it again on Blend, and if you ran it on Blend, run it on Large FFTs this time. Both tests stress different parts. Each test should be at least 8 hours.
Besides this, I'm starting to feel we're running into a wall, and the only thing left after all this would be to start swapping hardware.