another computer will not recognize iphone thread

i'm trying to JB a 2g iphone but the computer will not pick it up. It will charge with the wall charger but not with the computer. However, the computer will recognize if and only if i hold the dock connecter to the phone very tightly. If I let go it says that the device is unplugged.

 

yes, i should have included that in the first post, sorry. Also note that this is note my iphone. I have a 3gs. You think that the different cables would matter? I successfully jailbroke it just now but i had to hold the charger in which resulted in a sore arm