Just another LAN problem with a twist

Its another busy week, one problem I encounter that I want to post here is about a LAN connection problem on a problematic RJ45 connection.

The problem indicates a no connectivity even if LAN cable is inserted(take note that this is a previously working connection), on the first look of the situation there are possible scenarios that may have caused it, it maybe cause by problematic LAN cable or RJ45 connection,a LAN card problem or perhaps a software problem.

The first thing I did is checked and re-crimped(without replacing the connector) both ends of the LAN cable(visually connection looks fine) but problem still the same. I go ahead on replacing a different LAN card(PC is using a PCI LAN card), when I rebooted the system it detects the new hardware and load the corresponding driver but still a no connectivity exist. What else is left, a cable or a software problem? this will requires a lot of work if I will replaced either of the two, so I look for other ways to confirmed the problem.

Before I continue, I just want to add more info about this particular problem, the problem occurs after the two PC has switched location.The reason I include this fact because this will be bring twist to the problem. I re-inserted that problematic cable to another PC, it's actually the one being previously switched and to my big surprise the connection did work fine.

I re-insert both PCI LAN card to the other PC, still no connectivity exist. The LAN device used on the particular PC where the particular connection work is on board(strange?), I replaced both RJ45 connectors on the problematic connection and connectivity backs to normal.

Conclusion
A not so good RJ45 connection exists from the very beginning the LAN cable was installed its just it works on the original PC which is using an on board LAN, the problem was only discovered when the user switched PC.