As many others my gps would constantly search for signal. Here is a fix I found and it worked.
First go to settings, applications, manage applications. Make sure you are on the screen view with ALL apps. Go to maps and uninstall all updates. Then go outside and while pointing your phone towards the sky go to settings, locations and security and select to activate your GPS signal by selecting the boxes. Continue to hold phone towards sky for about 1 minute. My GPS now locks on again and turn by turn directions are back. Hope this works for you. When doing turn by turn directions you do need to be outside to get the lock on gps.