Why does the network disappear on Windows XP?

Question

Computer networks

Computer running Windows XP, integrated network card. After switching on, it works for 5 minutes, sometimes 30, and then the network does not see. Checked for viruses - clean. I bought and installed a network card in the PCI slot - for an hour I did not lose a single packet and again the network does not see. The integrated network at the same time blinks very often with the indicator, and on the switch the port in which this computer is turned on also blinks often.

Answer 1

the network does not see, but receives the address?
increased the number of half-open connections?