Help! No wifi or sync! Unlocked iPhone (using T-Mobile) has 3.1.3 firmware,

[Mrs B]

modem 04.26.08. Any ideas? Hope the answer to this problem is a simple one... My unlocked iPhone 3G has no problems with calls, texts, or pic messages. But the wifi and sync are not functioning. Someone told me to reset Network Settings, and right after I do this every time, the phone finds nearby wifi and connects for a minute or two. Then it completely stops and the T-Mobile upsell "web2go" message pops up and tries to sell a data plan (which we don't want and don't need to use wifi). After this, the error message "unable to scan for wireless networks" pops up. Also, the phone does NOT sync to my iTunes library. Phone appears to be syncing, but the device rejects the sync and says "canceling sync", and an error message pops up on my computer saying that the device timed out. All our other iPhones and iPod touch devices work fine. I have a feeling this is due to the 3.1.3 firmware update, and how it works in tandem with the 04.26.08 modem firmware. On a side note, my battery life is not good either. Any help, insight, or recommendations would be appreciated!!