the solution is here and i am talking from experience. it doesn't apple to dota only but apply to most of the games. if u suffer a sudden drop in fps then do this.

1. most probably your laptop or desktop fan or heatsink is all clogged with dust. time to give it a good clean.
2. it's your graphic driver. always use the latest graphic driver and download it ONLY from your manufacturer site. never download it from any third party sites, torrent or anywhere else.
3. lastly, if all above is done and u are still suffering from the drop in fps in dota only and no other game, it's time to do a clean install of your warcraft. if it still happens to all other games, might be a bad capacitor in your mobo or your harddisk is showing signs of failing.