So you're in a different apartment of the same building, and now you're having problems? It could just be a problem with the Ethernet wiring to your new apartment. In fact, it seems HIGHLY likely to me that you're running 100BASE-TX and your transmit pair is patched wrong.
If I were you, I'd try a different Ethernet jack in the apartment (if you have more than one), or ask my next-door neighbor if I could run a test using their jack.
If you all connect to the same switch and can see your neighbors' machines, I'd try a performance test between your apartment and a neighbor's apartment and see if I see the same performance problem in that case (with no WAN link involved, just in-building Ethernet).
What Ethernet link speed is being autonegotiated? 100 megabit or gigabit?
If this is just 100 megabit Ethernet (100BASE-TX, two pairs of wires), it could be that your transmit pair is terminated incorrectly, so that your two transmit pins aren't actually patched into a twisted pair of conductors in the in-building cabling. That could be destroying your Tx signal integrity, causing high bit error rates (and thus high packet loss) on Tx, while not affecting Rx much.
If this is gigabit Ethernet (1000BASE-T), then it gets more complicated, because all 4 pairs are in use, and all 4 pairs are bi-directional. It's harder to see how this could be a wiring problem. However, if it's still somehow a wiring problem, and the problem happens to only be on one or both of the pairs that aren't used for 100BASE-TX, then you could possibly work around the problem by forcing your router (or your PC) to only negotiate 100 megabit.
Who owns the wiring? Your landlord, or your ISP? Who owns the switch that you connect to? If you can get the landlord to let you access the wiring closet, you could run some more, very useful tests by finding your port on the switch and seeing what performance you get right there directly connected to the switch while standing in the closet; changing out the patch cable for your port with a known good cable; and inspecting how the wires for your apartment's drop were terminated onto the back of the patch panel, looking for poor connections, incorrectly mated pairs, or rolled pairs. You could also look to see if the wiring from the patch panel to your apartment is at least CAT3 (voice grade, but still twisted pairs) for 100BASE-TX, or at least CAT5 for 1000BASE-T.
In your apartment, you could open up your Ethernet wall jack and look at how the cable was terminated there, looking again for loose connections, mis-matched pairs, rolled pairs, and the appropriate grade of wiring. Be sure to study up on EIA/TIA 568 pinouts (especially 568b, which seems to be the most common in the real world) so you don't make bad assumptions. Pin 3 and 6 are a pair, not 3&4. Also, pins 4 and 5 are pair, not 5&6.
You could also check with your landlord to try to get a sense of how the structured cabling is layed out / physically cable-routed through your building, to make sure the run between your apartment and whatever switch you connect to is no more than 100 meters of cable.
If you have access to a fancy cable tester, you could do some nifty quality tests on the cable. But beware of the simple/cheap pinout testers that only test the pinout; these can detect rolled pairs, but not mismatches where two pins that are supposed to be on the same twisted pair are not actually on the same twisted pair. I've seen plenty of questions here on SuperUser where someone thought their cable was fine because they'd used a pinout tester, and it ended up being wired where 3&4 and 5&6 were on twisted pairs together, instead of 3&6 and 4&5.