I have to be honest, I've helped a LOT of people jailbreak an unlck, both in real life, and here. And I've never heard of your problem, man. On my own iPhone, I've used 6-8 T-Mobile SIM cards, and never had this problem. Regular accounts, smartphone accounts, prepaid accounts, dumbphone accounts, and never once have I come accross this. I'm sorry that I can't help man.
I happen to know for a fact that blacksn0w works well on both the 3G and the 3G, so it isn't a software error. I think it may be a hardware issue, where there's something wrong with your antaenna. But don't quote me on that.