I'm not sure a "good hard thump" is really such a good idea, even if you may think there is something loose. Don't forget that filler neck on the display [sad]

If you get "power cuts" both on DC and AC supplies, then it does tend to imply the player may be at fault. But to eliminate the obvious, I would be inclined to:

- check your in-car harness wiring is still in good order, with no loose joints anywhere. Look at voltages too, and with the ignition on and off.
- check the power adaptor at home, again for voltage, but also for temperature when loaded. Verify with a loaner power brick if you can to eliminate the brick.

I have once seen a hard disk failure where it would short out the power lines and cause fuses to flip. I hope this is not the case with yours. How old is the disk itself?