No Service after Jailbreak 4.2.1. PLEASE HELP!

Jailbroke my iPhone 3g to 4.2.1 using pwnagetool.I have 6.15.00 (iPad FW) and my problem is i have 4-5 bars when i turn the phone on or come out of Airplane mode. Then after about 30 secs, it drops to No Service, Iv'e tried it all people.

Toggle 3g & Data roaming
Re jailbroke
Removed sim & Tried other sims.
Installed/reinstalled Ultrasn0w.
I'm seriously at a loss.
I can say that I have Searched for over 5 days looking for the solution, and still have found nothing.
Please Help!