Mikrotik Ping Request Timed Out

Hi,

We are using Mikrotik RB 450G, we have an issue with our NAS Regarding ping request timed out… for example

One of the user have package 1mpbs, when your in ping gateway at normal browsing time the ping reply is fine when ever the user downloading or using full bandwidth at that time the ping reply is not fine there are so many request timed outs once the usage get down again the ping reply is fine…

Is there any rule we can assign regarding this so that user can use full bandwidth as well as good ping replay form gateway address..

Thanks & Regards
aztech44