2006 iMac screen goes black after a few seconds

I tried to restore a really old iMac (Late 2006, 24"), and after changing the SSD and putting the RAM back in place, I noticed that the fans were spinning up at max speed (probably a problem with the thermal sensor that was previously connected with the hard drive) and when I installed Linux Mint (unfortunately I don't have the original Mac OS X Leopard CD), the operating system usually only works from the CD in "compatibility" mode. If I try to start it from the SSD, the desktop interface appears for a few seconds before the screen (and only the screen, prob not the entire computer) suddenly shuts down as soon as I move an open window (not without noticing a slight graphical glitch in the taskbar and elsewhere).

• Concerning the graphics card (or chip?): I only cleaned the dust, but I didn't put any thermal paste back. Is it really necessary?
• The thermal sensor of the HDD is not connected. Do I need to install a resistor to solve this problem (if it has anything to do with ventilation of course)?
• There is no original Mac OS X system, and there is also no "recovery" mode on this iMac. Do I have to burn the original OS on a CD (like I did for Linux Mint) and then install it to use it correctly?
• I already reset the PRAM, by pressing CMD+Option+P+R and waiting for the boot sound to be heard three times.
• Wouldn't the problem actually come from Linux Mint (I have a patched version so that it is bootable from this iMac) ?

I look forward to your answers and thank you in advance for them.

And yes, this is translated. I already asked the question in French but i'm really not sure i was gonna get any answers (viewing from the lack of questions asked about this iMac), and I already looked up the forum rules (and didn't see anything related to language).