My iphone app working fine with 3G network.I am using apple Reachability class to detect the network availability
However while moving from wifi area to 3G,network is not available,or soo slow. Why its happening..any idea
It depends what transport protocol you're using. Because your device is moving between two different networks any existing connections (for instance a TCP connection you've started) will not work anymore (albeit without some fancy TCP handover magic). Because of this, background services in iOS that rely on a connection state may be trying to reconnect, possibly causing things to slow down momentarily.
Hope this sheds some light on things.