I just read that turning on WiFi (even though you're not connected to it) improves keeping lock on GPS. Don't know how or why that works but another member said it worked great.
When I'm having problems with my GPS, I use GPS essentials to get a fix on satellites and that has worked flawlessly. I put it through the test on a 5 hour trip to Baltimore using Google navigation and Sygic navigation. Kept losing GPS for hours. Started the GPS essentials app and didn't lose my GPS on Google or Sygic once.
The reason it runs when you're not using navigation is because there are other apps that also use it. Any app that uses location services (Facebook, camera, web, etc) will activate your GPS when needed.