To understand what the problem is with the network?

As a person has worked a long time to say that are most likely to logati port on the switch. Ask to praticola you another.
And in General, to exclude the provider can be fairly easy, tykni in the cable, a thread on another PC or laptop. (only pre-swap the MAC address of the 2nd computer on his own, and it may be banned (at least I would have banned))
I can still say that if you peratikos but that doesn't help, make sure is not lost if the signal is increasingly, if Yes, then probably somewhere in the cable frays, and soon he gets really bad.
Network card is usually either works or not. Half-measures are rare and usually behave the same as in your case.

Could it be problems with setevogo, or cable, or software?

That you yourself and answered your own question. Consistently start with the most simple. For example, change the OS for a while. Does not help — change the network card. It does not help, change cable. Everything is simple, "divide and conquer"

very similar to the problem with the network card, dig this way!

maybe the cable is too long and signal is weak.