COWON D3 has got a new Android 2.3 firmware update which fixes a lot of bugs and streamlines the OS, you can download the update for free, the current available version is 3.55 which can be upgraded from 3.53 firmware, this Android firmware for the PMP is the heavily trimmed version of the original OS and suits well to the device, download links of both new and old firmware are below.
Download Android Gingerbread firmware for COWON D3 PMP
Firmware 3.55 download link. You must have version 3.53 for this.
Firmware 3.53 download link.
Note: Take backup before the upgrade process.