Hello, agent007, and welcome to Geeks to Go!
and I'll be helping you out today.
It definitely sounds to me like a hardware issue. Is this issue happening on a desktop machine or on a laptop? It is possible that the card/chip is overheating which is why it works for a while and then just dies. If it is on a desktop, I would get a network card and pop that in the machine to test it. If this one works with no issues then the other network card or onboard networking chip is causing the issue. If it is on a laptop, I would check into getting either a PCMCIA network card, or a USB network adapter and seeing if that takes care of the issue.
Please let me know if you have any questions or need more help.