so my rx 5700xt nitro+ gets really warm und loud under full load. So i decided to undervolt it and set a custom fan curve. In the first minuets the games run and my gpu doesnt even hit 65 degrees. After a few mineuts all games just stop working. The screens turn black and the games are closing. I really dont know what to do at this point.#
What all have you tested/eliminated? What games? Define "Hot & Loud" "Full Load" and "undervolt" as you use it.
Probably most telling... does it crash under the stock profiles?
If your system meets or exceeds all requirements to use the GPU and it does not work at defaults then the card is defective and you should talk to the cards maker (Saphire) about support options including up to an RMA for a card that works.
You gave no system specs so it is hard to evaluate if a system incompatibility could be at play.
You can contact AMD support here as well if you like: https://www.amd.com/en/support/contact-email-form