Adhoc and static ip not working

Hi!
I'm trying to connect my A500 to a device which needs ad-hoc connection and a fixed ip address.

I rooted the device and did the wpa_supplicant hack. I get now a wireless connection to the device.

I now try to assign the fixed ip:
* With the Wifi-Settings
* With Apps like "Static IP Toggle" or "WiFi Static"
* With a root shell and ifconfig wlan0 192.168....

There is always the same effect, the IP address is not kept and the System says "Optaining IP Address..."

When I set the IP address with ifconfig I can even connect to the device with a Socket connection and send some commands... and get answers from the device. But after a few seconds the IP address is set back to 0.0.0.0 (calling netcfg). And "someone" is trying to get a new IP adress again (from a DHCP server?)

Is that a known problem?
How can I tell the system that the static IP is set and it's ok?
Is it possible to stop the service that automatically checks the connection?

Thanks for any hints in this issue!