Why can't my jailbroken iPhone connect to a wifi network? I have an iPhone 3G 16GB with iOS 4.0.2 and my wifi is not working, I tried to fix it with reset network settings and other quick fixes like that but it still doesn't work. Sometimes it sees wifi networks but I can't connect. I don't think is a hardware problem because I can see the wifi MAC address on the device.

Version: 4.0.2 (8A400) (jailbreak with redsnow)
Modem Firmware: 05.13.04

PS: On 3.1.2 the same thing. (jailbreak with blackrain)

Downgraded at 3.1.2, again...still nothing (jailbreak with redsnow)
It may still be a hardware issue -- I had a similar problem last year. But since you say that your phone sometimes does see a wifi network, there's hope that the hardware is okay.

First, follow the comments posted here to make sure you're running a completely normal OS without any hacks or modifications.  

Then, here's Apple's description of how to reset the network settings.

Lastly, if it still won't work, check my link above.