I've seen included in some other threads, complaints that the battery is going down, even when the phone is in the car-dock (and the car-dock is plugged in).
I've seen this blamed on both GPS usage, and other high power applications.
I've duplicated the problem, solved the problem, then duplicated the solution (at least for me, and my phone).
The problem is Temperature.
That big beautiful black phone is sitting with a large black face soaking up the sun.
I installed 'Battery Indicator' which provides battery temp information in the information display. I watched my phone over many hours this week as I played back podcasts, while running GPS, while running a live wallpaper, while having the brightness set to 'auto' in full sun so it was at full brightness.
When the battery temp is below ~42°C (Maybe 41, Maybe 43 or even 44) the battery appears to charge, or at least keep up with the above described usage.
When the battery temp is above 45°C (And I saw mine go as high as 59 at one point) the battery does not charge as fast/does not charge, and appears to lose power (due to temperature induced voltage drop) at an accelrated rate as the temp goes up.
It was so bad at one point that my phone was showing 50% battery, and then shut down completely due to low voltage (not low charge mind you... but low voltage because of the heat)
Solution 1: Angle the phone in the car-dock to be at an angle to the sunlight if possible. Don't let the sun hit the back full on for more than an hour-90 minutes if possible.
Solution 2: Angle an air conditioning vent in the general direction of the phone.
Solution 3: Mount the car dock elsewhere, or more towards you and away from the angle of the glass (and hence under the cover of the top of your cab/passenger compartment).
Solution 4: Pull the phone out of the dock once an hour and hold it in front of an air-conditioning vent and chill it down.
Solution 5 (Untested): Take some cardboard and craft a sun shield and tape it to the back of the car dock to provide the phone some direct shade.
Hope this helps -