I'm glad it's fixed; but it is strange. The modem is a separate hardware device and nothing that you change on the PC should effect it connecting to the ISP.
The the PC is trying to send info when the modem is connecting. In which case the isolation test would have found it as the modem would have connected before the PC was turned on.
I wish we could narrow down to exactly what the issue was/is. BUT if it works mate, I'm as happy as you are!
. Just continue to monitor it, you know what to do if the problem reoccurs and I'm always happy to help!
Enjoy your XMas.