I have had the clients on all my pcs after 3 weeks or so the client gets unistalled. We have checked that there is no policy uninstalling it.  Any thoughts or suggestions?

Nothing is ConfigMgr will initiate an uninstall of the client agent.

Are you talking about the actual client agent or the "client" column in the console being set to "No"? These are two very different things.

client on console is set to NO

I also checked a couple of computer and it looks like there is no client on the pc as well.

As Jason already mentioned: there is no mechanism in ConfigMgr that would truely uninstall the client. Examine the eventlog and ccmsetup.log (%windir%\ccmsetup\logs). It will show you at least when it was unintalled.
The only way to change the flag from "client = yes" to "client = no" is by using the maintenance task "clear install flag". Is that enabled at all?