Strange stuff with my PC

Hello there, I'm new to the clan, wanted to say hello...


Also I've encountered a problem with my PC.
I changed my PSU 20 days ago, I had some Chinese 550W PSU, everything worked just fine, but then decided to upgrade my GPU from HD 6950 2GB to HD 6990 4GB, I've got it basically for free, just paid for the shipping.
There was not enough power so I also added a Silverstone 750W certified PSU.

Now for some unknown reason, my PC just shuts down randomly and then it wont turn on, just keeps restarting, wont even boot.
The trick is, I need to remove the AC power cord from the PSU, discharge the PSU, and then again put it back in, and it boots, works perfectly, War Thunder now runs on 140 FPS on max with AA and other shjit.
I took out the PSU and did a stress test on my friends PC runing R9 290x, everything worked with any restarts or shut downs, so it ain't the PSU.
I put it back into my PC and the problem came back again.....Also used his 1000W PSU on my PC and no restarts...

Any of you know what it might be?

Since you tried a different power supply with the 6990 without issues and have tried the new power supply in a different computer without issues, there may be an interaction/poor connection between the New PS graphics power connectors and that particular 6990. Have you tried running the 6950 with the new power supply might help confirm theory that that PS and That specific 6990 have a connector/interaction issue? Try gently cleaning the power connector posts on the 6990 with IPA and well as carefully examining the PS power connectors for any possible FO (foreign object/dirt/contamination) and clean if appropriate. Of course do all of this while properly grounded to prevent ESD damage (sorry the 30+year electronic mfg engineer seeping out)

Dont have the HD 6950 anymore, I gave it to a friend of mine, he want's to try and unlock it to HD 6970 and run 2 of these in crossfire (think he bricked the GPU, told him that HIS IceQ-X cant be unlocked)
Hmm, The PC case is squeaky clean, got all kind of filters in there and the CPU is water cooled. I maintain it like every 3 months or so, make a full clean, change the thermal compounds etc...
Also checked for damaged cables leading from the PSU to mainboard, I've put electrical duct tape on all the cables, it didn't help.

But

I think I know what it might be, I also purchased with the new GPU a keyboard from MS, its some illuminated USB keyboard. Maybe the lights cause a short or something, dunno.
Hooked my good old IBM 25 year old mechanical keyboard now to test my theory.

Here in Bosnia they still sell Pentium 4 and C2D/C2Q for like 250-350 Euros :D :D :D
Its hard to find any good service for the PC

Update*

Turns out the keyboard was causing something, like a USB power draw overload or something, went tru all the trouble for a stupid glowing keyboard.
Also this IBM works great.... Found it in a garbage can a few years ago.....