SLI issues

[Calranthe]

Hello,
My Gaming pc:
i7 4790k stock speed
H100i corsair cooling
16GB Team Group
Gigabyte G5 Z97x Motherboard
SuperFlower 1000w platinum psu
970 GTX Galax graphics card stock clocks
Sennheiser PC 363D sound system
Iiyama Prolite E2773HS monitor
Intel 480GB SSD
4 TB HD.
Windows 10 64bit home premium.
Corsair 750D case well ventilated.

All drivers updated, recently I managed to get hold of a second 970 exact same one Galax 970 both running same clocks.

Physical installation went well, found an old sli bridge, added the second power cable to the psu, booted up pc and turned on sli in the control panel.

Everything seemed fine and then the instability started to appear it can happen in a game like Witcher 3 and Dungeon defender 2 or on the desktop or web browsing and is best described as some one slamming the graphical breaks on:
Frame rate starts to stutter
Frame rate slows
PC slows
Sound crashes
Pc freezes
After a while screen may go black and pc reboots or Complete lock up until a 10 second push of power button or windows 10 access violation blue screen or nvidia driver stops working but mostly it is the complete freeze and then I need to power down pc.

Heat on first card usually sticks between 50-68 on second card it goes up to 75.

At the moment I have taken out the second Galax and ordered a new sli bridge incase the old one is faulty.

Any advice is very much appreciated want to get my rig stable before fallout 4 comes out

 

[Calranthe]

Everything is at stock speed, bios updated, tests with each card separate show no instability, games run fine as does heaven benchmark.
Motherboard bios is reinstalled, will do a fresh install of drivers once I put the card back in.

 

[Calranthe]

Boards are back in testing beginning now, I have left the bios at defaults (since bios update).
The sunflower 1000w has separate modular fittings for each connection (love it) so no mistakes made there but it never hurts to check.
Have speed fan on my desktop normal temps on fans.
Ran the unigen heaven benchmark 1920*1080 max AA, Max Detail, Max Tesselation) maximum temp reading was 78 and score ended up as 2450.
Going to play some Witcher 3 and see what happens.

Oh and cards are in the recommended slots for dual sli.

 

[Calranthe]

Update 10 minutes of Witcher and screen went black for 5 seconds then bad tearing on the screen and temp of one gpu has shot up to 85c. is that a bad temp ? other one is staying at 67c

 

[Calranthe]

I am going to go back to a single 970 it just isn't worth the instability I am getting, will try again but the machine is used 12-14 hours a day can not have it going unstable.

Any new advice I am happy to try, thank you for the ideas.