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My co-worker and I have been testing various solutions to the slowness of 3.1, IMAP vs Exchange Sync and other areas of network usage. Last night he discovered a real speedup improvement by removing a tethering profile that we both had from trying a tethering hack on our phones (3g and 3gs) about 4-6 months ago. By removing this old tethering profile, it seems that all network access has significantly sped up. Neither of us have had the other reported issues with reboots, freezes and dropped calls (other than the norm). Anyway, go to Settings, General and see if you have a Profile option. Remove it and reboot your phone to test.
Good Luck
MacBook Pro 15 Mac OS X (10.5.5)

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