75hz with DP, not with DVI-D

Hey guys, I just picked up a LG 34um67-p, and have a Nvidia 970, and had a question about refresh rate on the monitor. My nephew gave me a Displayport to DVI-D cable he got off amazon, doesnt need it anymore. I hooked it up, but the monitor showed no signal, so I did the HDMI trick, had both cords into the back of the monitor, swapped which was display 1 etc.. Restart the computer, go back into my Nvidia control panel, bump up the max resolution, with 75hz, apply and works like a charm.. Today, I saw a DVI-D cable, purchase it, come home and hook it up ( Wanted to keep the cable clutter down ) and monitor turns back on, goto the Nvidia CP again, but the hz is stuck at 60.. I create custom resolution like i did last time, but when i bump it up to 75hz, the letters are super fuzzy, and distorted, very unreadable. Is there a major difference in going from DVI-D to Displayport / DVI-D connector, or should I just get a Displayport cable and be done with it.

Displayport cable and be done. I have the same exact monitor. Set to 75hz. Text is fine.

Awesome, thank you, ordering one right now..

which GTX 970 do you have? if it has more than one DVI port, try the other one. sometimes video cards will have one digital and one analog DVI port. you'll have to use the digital one in order to get the higher refresh rate, analog is limited to 60hz.