Dell Latitude E6400 LAN-Card issue

Hi.

I recently inherited my fathers old Dell Latitude E6400.
I work as an IT-Apprentice, so i thought the task of refurbing the Dell would be an easy one.
So far i've been right, but for one thing.

The LAN-card will work and connect for several minutes after the computer has been shut off for 24+ hours. After a couple of minutes it will turn disabled, and i can't enable it. couple of minutes after that it will disappear completely. From Device manager and all. I've tried flashing the bios and clearing CMOS, installing every driver i could find from both Dell and Intel, but nothing seems to do the trick.

I was hoping for some help finding out why.

 Thank you.