So i am quite happy i managed to find a solution that works. I still run the undervolt for other things and some other wallets but not for BTC core.
It sounds like you've created a hardware problem by undervolting. You may not notice the problems in other applications, but they could still occur.
I'd say improve your cooling instead of using hardware outside it's specifications. For your next computer, consider a processor with lower TDP if possible.
So that's why Bitcoin Core gives a warning when started for the first time:
This initial synchronisation is very demanding, and may expose hardware problems with your computer that had previously gone unnoticed.