If you have an iPhone 4 with baseband 1.59.00, you will encounter a major compatibility issue with ultrasn0w 1.2.8 and iOS 6.1.
You will notice the following symptoms:
- It cycles between “no service” and one bar signal with 3G.
- Every 20 seconds it pops up one bar for your network and then returns No service.
Unfortunately, there’s no fix for now. Even the ultrasn0w fixer won’t do your iPhone 4 any good.
See the comments section below for possible fix.
Your best option for now is to downgrade your iPhone 4 back to iOS 5.1.1, assuming that you have the SHSH for it, but most of you should have it saved on Cydia/Saurik’s server already.
Of course, the best solution is to get your iPhone officially unlocked so you won’t need to deal with this type of problem in the future.
Meanwhile, let’s wait for a a fix from the Dev Team.