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.


my edimax has been stable ever since I changed my network settings

[code]auto lo
iface lo inet loopback

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