I use Windows and Linux Mint 18.2 KDE in dual-boot. My GPU fan is not working, but I've managed to keep it somewhat cool using my electric fan and removing the pc case door. In Windows, with a lot of softwares running at the same time (Evernote, Quoll, Steam, Chrome, Inkscape, Gimp, etc) and the electric fan turned on, the GPU temperature stays between 35ºC and 40ºC. But when I logon on Mint and there's no software running - with the electric fan turned on - the GPU temperature goes to 75ºC. Is there any setting I can disable (or enable) to prevent this overheating?

Yes, my drivers are up to date. I had to add a ppa for the mesa drivers.