I've had the same issue with my new 825EU. As you say, it only happens when a route is planned, and only then after a spoken instruction. We've turned off the "power down when external power removed" option, so on ours, after the spoken instruction, the screen dims and the unit then says "Charging battery" then goes back to normal.
On ours, it happens when the car is stationary (so no vibration problems) or moving and also with two different power leads in two different cars, so it does seem to be something in the unit rather than actual power supply.
As ours is only a few days old, I've just sent it back for a replacement. Hopefully the new one won't do this!
Cheers, Steve