Hi,
First thing I would do is check the cables to see if they all have guaranteed clean connections. You will need a cable tester to do this. After that I would try a different switch to see if that is not a problem. It might sound a bit strange but when you took the cables out of the PC's and replaced them with new cables there is a high chance some fluff got into the nic socket and is causing problems. There is always fluff and dust down behind the PC. Just as a side, I have found that a USB powered torch is great when working behind PC's. You can get them on ebay for next to nothing.
If you want to see what is going on on the network itself then get something like ethereal which you can use to do packet level checking or even MS Network Monitor 3.
Cheers,
Lee