Win vista internet connection

Hello,
sir i hav windows vista on my laptop pc.i hav a LAN .some times my internet is not working ,while the Local Area Connection is working but not connected to internet.if i disconnect the LAN connection and shift my pc to another LAN port it gives same problem.but some times suddenly it is connected to internet ,it is a strange behaviour of pc.
please help me.
regards.
ARIF GARDEZI