OK @Dschijn, thanks for the suggestion. I tried with a different, 7 year old 550W PSU, which on the first few tries seemed to last slightly longer (5-10 seconds) before cutting out. However the last few tries it's the same story with this PSU: As soon as the load increases to handle 3D graphics the graphics card cuts out after a second or so and the system reboots. The total draw of my setup (Zotac 970 + Akitio) never exceeds 205 Watts, so I'd be really surprised if the Nexus PSU can't handle it. Zotac recommends a 500 W PSU for this card, but that is for a full system, while I am running only a graphics card with an Akitio box.
As the Nexus is multi-rail, it could be that the OCP is triggered, but I am not sure why it would happen under these relatively limited loads (I am running an ITX sized 970, not a 980 or something). Am I correct in understanding that I do not need to make any changes to the Akitio to increase the power limit of its PCI slot? It runs fine using the original adapter, but that adapter is not designed to handle much more than 25 Watts, right?
Are there siginificant risks if using Akitio adapter + PSU for a longer time? I am really stuck here....Thanks for the help.