Yes, 339Y, or Yes 3339Y?
Assuming i5-3339Y, here is your processor and its drivers:
The graphics driver supports upto 3 monitors.
I do not know what your laptop or desktop model number is, or why you are running in safe mode. Can you provide more detail.
Safe mode is not production mode. And, if everything was fine in safe mode, you would not be here.
Did the wifi ever work ok (disregarding safemode)?
What wifi do you have?
Is your machine a laptop or desktop? What model number?
If laptop, did you add or upgrade the wifi yourself?
Have you considered doing a fresh install of Windows 10 (using your W8.1 Pro key)?
The computer is 7 yo. It worked for the first few years yes, then it had a little wifi problem: the connexion would stall for every machine in my home every 90 minutes or so. I changed the modem and tried another network card, to no avail. Note that it only happens from home. A fresh install could work, but I have very little room on the 100go HD and so I would prefer to keep things as they are.