I have a recurring problem with my iPhone 3GS: every few months my cellular network won't work.

The reason for this is that I installed a fake APN when on IOS 3, which disabled my GPRS and 3G. Because I bought the phone without a contract, I did not want to use it's cellular data network for internet.
After a while I changed my contract so I could use internet. But at that time I had already updated to IOS 4.
The problem is that IOS 4 removes the option of installing an APN completely, so I couldn't disable my fake one.

If I try to install an APN it states that one is already installed.

A temporary fix for this problem is to restore the iPhone as new and delete a file (8218978e4ab0a48035bb92653145a6be872ea858) from you backup folder. Then restore from that backup. That file contains the faulty APN information.

Only this occurs randomly now and it's starting to get annoying.
And yes I do not want to set my phone up as new, that's why I'm asking for help.

I thought about downgrading my iPhone to IOS 3 and then removing the APN. That might fix it, but I don't know how to downgrade, remove and upgrade without losing any data or jailbreaking.

Can you guys offer me any thoughts about this issue?