I had a similar issue when attempting to downgrade Cisco 3600 Series access points to Autonomous, spent a decent amont of time trying to TFTP from my PC but kept timing out. I eventually just TFTP'd the image to the Cisco 3750X switch and then TFTP to the AP from there. It's a workaround if you have Firewall and Antivirus on your PC. Hope it helps.
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