Why am I lagging with my new GPU?

[simonsonter]

In december 2015 I got myself a new graphics card.
Radeon R9 390x

Should be a good graphicscard. Although it started my mxsim to lag really bad, especially outdoor tracks. I didnt do anything to the graphic, it stayed the same as with the old card. But it still started to lag. So I tried to lower graphics and it still laggs. It feels like its not the graphic thats the problem, since I play some other games in Ultra high graphic. What is the problem then?
My other graphiccard was Geforce 560 Ti.

[Wahlamt]

Are you playing in windowed mode? Are you having vsync enabled?

[simonsonter]

No, fullscreen. I have no idea if Vsync is on or off

[aeffertz]

You completely removed all the nvidia drivers and software before installing the new graphics card, right?

[simonsonter]

yes ofcourse