I overclocked my pc and now after a few minutes it black screens and fan speed greatly increases.

[cpr05]

It was idling at 72c so I thought it was a good idea to roll it back. So I reset the jumpers on my board and I'm still seemingly overheating?

G3258 previously ocd to 4.2 on stock cooler
R7260x OCD

 

[Yamitime]

Stock cooler will be your nemesis. If your overclocking anything you have to get a suitable cooler.
Did you check temps before you overclocked as a baseline?