iPhone and WiFi/3G Switching

Up until about a month ago, when at home my iPhone quickly realised and switched to WiFi, instantaneously.

For some reason, of late, when I got to browse, check mail, read news etc, it will stick to 3G until I cancel the operation and try again, then it will start with 3G and after about 30 seconds switch to WiFi.

Anyone else experienced this?

Not a big deal but a little annoying.