network path could not be found

i have install SQL server SQLExpress 2012 in my local laptop. my problem is everytime i run an asp .net c# page, i get error 'System.ComponentModel.Win32Exception: The network path was not found'. i've check everthing, firewall, connection string, SQL Server configuration management but nothing works.. the connection is not stable..sometimes it can connect, sometimes it cannot connect.. how can i fix it.. below are my 2 connection string that i have tried in my web.config. tq so much...