RTX 5090 tripping 1200W PSU

If I use LACT to limit power to 400W then all works well, but >420W will trip the PSU.

Seems an awfully low limit to have to set. its a ATX 3.1 PSU so should be fine with brief high power spikes. I wanted to set a negative GPU voltage boost, but I can't do that (with LACT at least). Temperatures all look fine immediately before power off (GPU around 70C), CPU is basically idle.

Any way to set negative voltage offset on this rig?

I am using comfyui and generating video running on Ubuntu. If I set power at anything over 420W after a few seconds the PC switches off with no warning. Also occasionally switches off under windows (11) as I start a game.

PSU: 1200W be quiet! Dark Power 14, PCIE 5.1, 80PLUS Titanium, Quad Rail, 100A, ATX 3.1 PSU 8I6LS

GPU: MSI RTX 5090

Mobo: MSI x870e gaming plus wifi

CPU: Ryzen 9950X3D x 32

RAM: 128GB

nvidia drivers 595.91.07

OS: Ubuntu 16.04 26.04 (linux 7.0.0-34)