i bought a second hand iphone 3GS 8GB the other day it was all fine locked to O2, so i JB it thru JBme.com and then unlocked it using U.snow, so put my orange sim ni the phone and the signal comes in at a healthy 5 bars, if i leave the phone then its fine the signal stays for a good few hours but if i start using it for messaging, downloding apps etc the bar slow goes down and then it says the network is restricted. i thought it might of been a case that the phone was stolen and it was reported stolen but if i take my sim out and put it back in the phone it goes back to the start again

any ideas what it could be? could it be my orange sim is too old ? its about 11-13 months old the sim itself?