sorry, probably a dumb question

[Squirrel240sx]

i tried searching all over but maybe i'm searching for the wrong thing
my iphone 3g was downgraded to 3.1.3 from 4.0 (too laggy) and now won't allow me to install ANY other firmware. i've tried 4.0.2, 4.0.1, 3.1.2, and the phone just reboots when i try upgrading in itunes. after the process it's still on 3.1.3 and itunes gives me error 21 could not complete upgrade or whatever. thoughts?

it also gave me problems when jailbreaking 3.1.3. redsn0w wouldn't work, it would just reboot to a black "off" screen when i tried putting it in dfu mode during the jb process and be stuck there until i did a restore in itunes. i had to use a friend's sim card to get past the activation screen and jb over at jailbreakme.com. i'm just a little confuzzeled. coming from a touch pro 2 which, imo, was extremely easy to unlock & upgrade. i don't get all this iCrap.