Like others have asked when you say it looses power. Do you mean it reboots or that it turns off?
The flash memory on the Avic F series becomes corrupt and has to be reformatted every few years. I've done it twice in the five years I have owned my F700. You have to get into the service menu and format the flash memory, then reload Windows CE, then the Pioneer frontend, then the maps.
Here is a thread on Avic411.com with detailed instructions.
Solved the FormatMBR /SDMLC and MapDBinfo Error 0X0000 (Also the Major Fix) - Problems/Troubleshooting - AVIC411.com