First of all, DoTA is only about 20GB.
Secondly, your download constantly restarts (well, backtracks; progress jumps) because when your computer power goes out (assuming it's the instant-powercut-and-everything-goes-black-and-the-system-suddenly-shuts-off type of power cut), Steam doesn't have a chance to properly finish finish downloading the file it is patching. Because of this, when the system restarts, Steam checks for the progress and will think the file it was patching is corrupted (not properly and fully downloaded). From there, it will restart downloading that file.
Usually, Steam downloads several files at once, which explains why there are big jumps.
Also, don't delete anything (not even use the "Uninstall" button in Steam) whilst something isn't downloading properly. Let Steam manage that; it automatically scans for any corrupted/incorrect files and delete/redownload/reaquire them.
Deleting/uninstalling will only mean it will have to redownload everything again. from scratch, which is the same as hindering your progress.
If you are constantly suffering power problems and cannot keep your computer on long enough to download all the files, you should consider not downloading DoTA as it contains lots of large files (and thus mean you'll be very likely to have a power cut before Steam manages to fully download a single file). Or instead, switch to battery power, if you have it.
Pausing the download and quitting before the power-cut (if you know when it happens) will also help as Steam will be able to leave off from the point that you paused it at.