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Check out following thread for illustration.
Downgrade from 12.4.x to 12.3.x
»Cisco 2621XM, Poor Performance with WIC-1ADSL, IOS 12.4
»[HELP] Cisco 877: connection drops when launching Emule
Upgrade from older 12.3.x to newer 12.3.x
»[Config] Cisco 2801 + WIC-1T problem
»[HELP] Cisco 7200 VXR uses up all of the I/O memory
Unidentified interface with factory-shipped IOS image
»[Config] Cisco router 867 what is the DSL interface?
Upgrade IOS image version for possible solutions
»Cisco 2821 ISR w/ Bad Ping, Error Inputs, CRC Errors & FiOS
»[Config] Adding 7920 knocks other phones off CME
Check out this FAQ for possible bug within the IOS image that might explain the phenomenon.
»Cisco Forum FAQ »Where can I find Field notices to confirm to during troubleshooting?