iPhone 4 switching back and forth between 3G and Wi-Fi every few seconds

I found this message when searching on the same problem. This only started happening to me recently. Normally I could leave my iPhone 4S connected to WiFi and it would stay that way. Now I am seeing that when I click a button to wake it up, it will say "3G" for a moment before switching to WiFi. This is happening on several different networks so it is not one particular router giving me trouble. If I leave cellular data turned off, then it obviously does not switch to "3G" but it does keep the WiFi icon displayed so it is definitely not losing the WiFi connection.

Something is definitely causing the 4S to switch to 3G when it is idle. I did try forcing a restart but short of doing a full restore, I am not sure what else to try. I have the "System Status" app installed and it is not showing any unusual applications running and I have killed all running apps just to see if one of them might be causing the problem without any luck.

UPDATE: A colleague also has an iPhone 4S that he got the same day as I did. I checked his and it is doing the same thing. If I leave it undisturbed for a few minutes, the 3G indicator will appear. If I press a button on the phone, it will say 3G for a moment and then switch to WiFi. Makes me wonder what it is doing. I may turn off the cellular data in case it is sucking down bytes behind the scenes.

time for reboot

Reboot the router, make sure the router firewall allows the connection etc.
I had to reboot mine because iCloud items on the iPhone were missing until I rebooted.

@Zzznorch - This is normal. iPhones always drop WiFi when in sleep mode after a few seconds unless WiFi is in use. Word is that it saves battery life. But it can also cause some eye-popping data charges from the carrier if you fall asleep listening to Pandora.

@m0rnings - DocMartin has a good suggestion. If that doesn't fix it for you, make sure you're not wireless "n" on the 5GHz spectrum. iPhones don't play well in that space. If your router can do 802.11g, or "n" on the 2GHz spectrum, try that and see if the behavior is better. Also, obviously, a bad password could cause the behavior too. Try removing the network under Settings, and add it back in.

@dotme: Thanks for the info on the network behavior. This is my first iPhone (was a gift) and am not familiar with some of the quirks. Prior to the iPhone, my iOS experience was based on an iPod Touch 3, then replaced by an iPod Touch 4 and an iPad 2 WiFi, neither of which exhibit 3G switching for obvious reasons.

I realize now that I was seeing the behavior because I had been leaving the cellular data turned off most of the time since I had gotten the iPhone. My 4S is with Sprint and I had been having such terrible network performance that I was relying on WiFi exclusively (an artifact from using the iPod Touch and iPad 2 WiFi devices - I have access to cable company public WiFi just about everywhere I go so have had little need to use cellular data). I only recently turned it back on full time not realizing the behavior started at that time.