Volumio B+ disappearing from network after some months


i use a Raspberry Pi B+ with volumio 1.55 for over a year now. It connects via WLAN dongle, I control it with an upnp app. After listening to music, i shut it down in the web gui and then flick the power off. Everything works fine, except regulary (about every second or third month) it won’t show up in the network after booting. It won’t show up as a upnp device, the webgui isn’t reachable, there is no active entry in the router etc, no way to contact it, os seems broken. Even connected via ethernet cable it isn’t there. Only solution i found is to reflash volumio and start from scratch. Then it works again for some months until the same behavior reoccurs.
I’m really stuck. I’m no linux guy and can’t imagine what goes wrong here. Can someone perhaps push me in the right direction?

Thanks alot,