I had an iPhone 3G that would give 1600 errors with any Jailbreak back in the WinPwn/PwnageTool days even though it worked fine with official Apple FW. A replacement phone worked fine with the same cables and PCs. Never figured it out and was only able to JB with custom firmware in REGULAR restore mode (not DFU). Never would have guessed except that someone with the exact same problem suggested it online. DFU is lower level with less to go wrong, so why would restore mode be more able to load CFW?! Go fig.