I've also had the rather unusual constellation where it was the clients bosses new PC acting funny after half a year.
Login OK. Email without attachments were mostly OK. With attachments almost never. Opening files from the server was flakey at best, mostly it would not work. Internet would work, but half the web could not display... Often also DHCP problems. Tried with adding a second NIC (LAN-Card) to replace the onboard one - still the same effects.
In the end, I plugged that PC directly into the switch, a floor lower. Everything worked. Testing another PC in the bosses office the same phenomens, bad connectivity...
We measured the in house wiring, which was by then about 10 years old. The cabel in the wall was screwed, either corrosion oder some other issue. After the electrician repaced the wiring, all worked and I could reactivate the original NIC and remove the PCI one.
A lot of us IT guys consider in wall cabling almost as sancrosanct, we never question connectivity. But sometimes it's just that! And I've also seen electrcians mount LAN wiring into a cabel-channel with broken copper sticking out of the side... (I only let the guy go AFTER he replaced the non working wiring he just mounted...).
It's not always our logic or the higher lever network stuff. Sometimes you can't rely on even basic copper - but the error is IN the wall, so you can't "see" it!
My 2 cents