From where i can download old versions of routeros for RB700 series. i want to download router os 5.4 for my rb750gl.

waiting for link .

You can't. It's not recommended to put old versions on a new RouterBOARD model, as it could make it non working. Some drivers are added in new versions specifically for new devices, also speed improvements and fixes are made.

actually my rb was come with 5.4 after upgrading on 5.7 i am facing a problem. link of problem is this

http://forum.mikrotik.com/viewtopic.php?f=2&t=55828

so i want to install back original version in rb again and want to check.

You can generally copy the download link and paste it into your browser, then manually edit the version number. You can pick up a few versions back that way usually. Otherwise your only option is to see if someone has that version saved somewhere that is willing to let you download it from them.