Datorn startar bara om när jag använder displayporten

hittade detta på nätet med en som har exakt samma problem som dig

"After much research it appears there are dodgy DP cables on the market. The true VESA compliant DP cable has no connection between pins 20. Pin 20 connects to supply on source and supply on destination. Therefore if pin 20 is connected and the mobo receives the supply voltage from the destination it goes into power-down mode for a few seconds and retries to output DP signal. If the DP cable is removed it will boot but when the 'bad' cable is reconnected, although it works, the two power-supplies will be in conflict and its unknown what will happen, long-term. The DP (Display Port) is also hot-swappable.
HTH "