I know connectivity issues can prevent the game from saving correctly but there might be something else going on.
Next time you get this issue, can you please exit the game and then reply here with your log1.txt file attached? This will help us isolate what the issue is.
Also, can you post your in-game name, the name of the city you are using and the slot it is saved in and someone will take a look at your city to see if there is any corruption on that end.
Hopefully we can sort this out for you soon.