Wi-Fi network unstable

I have a Raspberry 2 running Volumio 1.55 with EDIMAX Wi-Fi USB. I also have a DAC board on the Raspbery pi. I have disabled the wired Ethernet port to avoid the DHCP delay as I am using Wi-Fi connection most.

I found that the Wi-Fi connection is unstable. When I powered up the above setup, 50% of the time, I cannot ping or access web gui on the Raspberry pi. However, if I connect the HDMI cable and then power it up, it works most of the time. At first, I through it was a bad power supply and now I switched to battery.

When I cannot access via Wi-Fi to the raspberry pi but with the HDMI cable and keyboard, I found ping from the Raspberry is sometime OK but ping to the Raspberry is NOT. Any suggestion to trouble the problem is welcome.

regards
Derek

my edimax has been stable ever since I changed my network settings
/etc/network/interfaces

[code]auto lo
iface lo inet loopback

auto wlan0
iface wlan0 inet dhcp
wireless-power off
wpa-ssid network_name
wpa-psk passphrase[/code]