I just read in another forum about a possible fix. It has to do with right after connecting/pairing successfully there is a quick pop up about "always trust this connection" and you must check this before it dissappears.
I am pretty sure I did this once, and again early on the Evo would work in the car, but eventually stopped.
I had to go into applications/running processes, and I stopped anything I could see was related to bluetooth. I then was able to finally "unpair" the uconnect, and start fresh. After doing this, the box popped up and I was able to select "trust this" or connect automatically whatever it says, and I tried it 3 times successfully.
You cannot get the option in the quick popup unless you are connecting this device the first time. If it is not checked, my guess is it just disconnects after every attempt.
The weird thing is it still says in the advanced screen "paired but not connected" but if you look at the home screen at the bluetooth icon you can see the arrow's on both sides of the icon. Hope this helps.