Persistence and loadbalancing

sometimes the user been drop while entering the login info and sometimes been droped when he enters the site and try do browse internal links

Are you suspecting this is because of a lack of persistence at the app tier? In any case, it would make sense I think to have persistence at both tiers. The trick is that you have to have some value that comes from the client that gets all the way to the inner F5. So for example, do cookie persistence on the outside, disable SNAT on the outside, and do source address persistence on the inside. This of course assumes that client addresses are stable throughout a session. If that's not the case, then you could potentially create another cookie on the outside F5 and use that in a cookie persistence profile on the inside.