restore fails

Ive always restored my backups from “live” to my “dev” site but today its not working….actually causes such a load that msql crashes. even tried a restore of a few tables on live site and same problem. This is not good as we need backups obviously. any ideas? ive tried everything even more memory in php.ini but no luck.

After restore fails site is not responsive for a while then get 504-503 errors but eventually comes back and i have to manually delete .maintenance file from publichtml folder via ftp to get things back to normal again.