Why ismy iPhone 4s switching from 3G to 1x randomly?

I've called Sprint about the issue. Was told to dial/call "##update#" it initiates a reset of your phone with the local cell phone towers and helps me a great deal - especially if I've turned off my phone while traveling and turn it back on in new location (across state lines, etc). The phone get's 'confused' as to where it is so by resetting it 'finds' where it is again and what network(s) to utilize.

Simply turn wifi off. Your phone keeps checking for available wifi connections and that is why it continuously switches.

That was not sufficient for me.

I was not able to connect to the 3G when I was outside, and even though my phone showed connected to my wiFi when I got home, I could not pull up anything inSafari - just indicated I was not connected to the Internet.

I called Sprint who had me insure voice and data roaming was on and International was off. Then she had me go into General to Reset to Reset Network Settings. The phone rebooted and I had to enter the password for my network, and Internet access was back.

II have Verizon service with my (old) iPhone 4S, and have the same trouble. Verizon suggested a new SIM card, and they offer replacemeets for free; walk into the store and ask.

i called while I was researching, and ordered a new card from them in the mail.

over the weekend, I got the idea of the corrosion on the metal of the card, and tried to figure a way to clean of the "rust" and old stuff....and decided toothpaste would not be too bad....and guess what, I'm back to faster data, Siri doesn't give me trouble and the 3G stays on all the time again.

Bottom line, brush your SIM card regularly!

Thank you, this fixed my issue.