@Pichlo

I have tried the app you have proposed and got a tremendous freez - I could only take out the battery. The gsm was turned off and I was in app manager installing apps to stress the system.

Then I have uninstalled it, overcklocked to 1000mhz and went to app manager. Installed many apps and... nothing!

The bottom line is - this phone for some unknown reasons doesn't like te gsm to be switched of with apps. I think that the build in offline mode is accepted by this crazy N900.