Are you referring to Network Home Directories, or NetBoot clients?
The scenario you are describing sounds like NetBoot. User changes persist for the life of the NetBoot session, and are reset on every reboot. That's exactly how NetBoot is supposed to work.
If you are talking about Network Home Directories, those changes should be persistent across multiple login sessions.
we use Netboot ... yes but every User which is log in to the netboot client get an own Network Home Directory mapped.
I know that changes on the OS not stay persistant inside the Netboot OS. But changes on the UserProfiles, like default Browser or somthing else, are also reseted after the reboot. I think they should saved in the UserProfiles and stay persistant if the user log in again after the reboot?