You've *got* to power-cycle the modem when swapping devices or you'll get exactly the symptoms you're describing.
No idea on the MAC changing, unless there's some virus around that does it.
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I generally do not understand why it happened and what (if any) is the relationship between the Ethernet cardââ‚ ¬Ã¢â€žÂ¢s MAC address, the modemââ‚ƚ¬Ã¢â€žÂ¢s MAC address and IP assignments.
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NTL (for cable modems) don't care about your ethernet card's MAC. The modem's mac is stored against your account details. The IP address is given to your MAC, often for quite long periods. Changing your Ethernet card's MAC results in a change of IP.