My iPhone won't scan for Wi-Fi?

Its a iPhone 3G and it occasionally does read my Wi-Fi so there nothing wrong with my router because my iPod 4g reads it all the time perfect so why does my iPhone not read it all the time is there a app to fix this?

No, there's not, But your iPhone Wi-Fi mite be Burnt out, and needs to be replaced, and it's common for iPhones, i have the same problem with 3GS but now it works well.