my problem is sometimes ping is 6 or 7 Request timed out and reply 2 or 3 or 5 and browsing become very slow Although there are enough bandwidth … and when i pull all clients all thing become very good and ping perfect reply and it,s very hard to go to every client and scanning his computer and clean it because Numbers is very big over 400 client am waiting your reply >>> i will be crazy because of this big problem
Could be malware, could be dos attacks, could be ping floods, could be a host of stuff. Some of the filter rules here should help.
http://wiki.mikrotik.com/wiki/Firewall