iPhone 4 Carrier / Cell Signal Issue
Just wanted to share my findings in case it can help someone with a similar issue.
I recently updated my phone from 4.0 to 4.1 using Tiny Umbrella to prevent the baseband update. My phone is locked to a carrier different from what I use. I Jailbroke with Limera1n and then unlocked with ultrasn0wn 1.2 (repo666.untrasn0w.com). This is where I ran into intermittent issues with cell signal. It started out working fine and then I would loose all cell signal. When I turn on / off airplane mode I would get "Sim Failure". I had my sim replaced but that didn't help.
I ended up removing ultrasnow 1.2 and then installing ultrasn0w 1.1-1. It has been working good for about 3 days now.