Could you make sure the molex adapters are not running on the same line?
There are seperate lines for 12V and other voltages which could be heavily affecting your GPU.
The CPU (65W) + GPU (185W) + other components ( extreme RGB worst case sceneario ) [30W] means that your usage is usually under 250 W or so even under long periods of gaming.
When it crashes can you check if there's a mother board light that's generally blue?
This light usually means that the system doesnt have the required power and has entered power saving mode. Which also means that the PSU is .... fried.
Come back to me when this doesnt help or it helps.
If it doesn't help I could get you back on track as it helps narrow my understanding of whats going on in that poor rig.