I think stock speeds should be 400/800. Firstly gpu was set at 400MHz by default (by Inno), secondly RAMs are 2.5ns Hynix, so OC is measured only after 800MHz.
So it really "dies" at 405MHz, with both bioses. "Dies" means: stops, and sits back to throttle speeds, which means, it gets too warm, or sg. Stock cooler is pretty small, and does not cover vrams. Other Inno 5900XT users can OC to about 420 with this cooling, which is still a lot higher than mine.
You can be right: - cooler does not fit to gpu perfectly
- gpu is faulty (what is PSU?
I'm sorry for this amount a questions!