So I have an iphone 6 plus and i can pair the phone to my car to make a call until I turn off the engine and if I drive later it doesn't connect to my phone unless I pair the phone again. What is the problem?

I have a 2008 lancer GTS with Navi and bluetooth.

 

I also have a 2008 Lancer GTS and Iphone 6 plus and for some reason my Bluetooth sometimes works and sometimes not work. Its been not working more recently. I would push the command button and nothing. So I have stopped trying and just used my speaker phone to do calls. Then while driving without me touching anything it will connect to my Bluetooth. There is no rhyme or reason when it connects or not. I pretty much have given up trying, but if anyone has an idea how to troubleshoot this I am happy to know the resolution. Thank you.