You can try a firmware upgrade to 4.13, which you can get from ftp://ftp.dlink.cz/dir/dir-615/driver_software/DIR-615_fw_revD_4-13B01_all_en_20101201.zip (you need the .bin inside)
Or, you can try the custom DD-WRT firmware, which is reportedly more stable. See here: http://forums.dlink.com/index.php?topic=13599.105
Update is done from the D-Link firmware interface under Maintenance > Update.
To get mode N go to Setup > Wireless Settings > Manual Wireless Network Setup and check the 802.11 Mode setting. You can set it to Mixed 802.11n and 802.11g and save settings. If that doesn't solve it, set it to 802.11n only.