How to resume interrupted downloads when it stops and starts during the download process?

I have a 2011 25inch iMac desktop and it when I download a game app from the App store, it keeps on stoping and starting at certian points (these are random and do not stop in the same spot if I restart the download) Ive only gotten as far as 6.22GB out of 8:55GB before the download progress stops and goes back to the begining all by itself. I have plenty of space on my hard drive to install it so why is it constently doing this. Its taken me a week and a half to get to about 6.22GB and its still not letting me finished the instalation. Also why is the Calulating times keeps going up and down some times it says like 1 hour and 55 minutes when its progressing but then when it stops it goes all the way up to like 15 days, 14 hours and 60 minutes until it starts up again and then it goes back down to 1 hour and 54 minutes when it starts up the instalation progress again. The Game app is Trainz Railroad Simulator 2. 😕😮😟