I used geth fast sync mode before, but it kept stopped and when it stopped, I had to delete all synced data and start over again. So I changed to "light" sync mode, and it worked well for a month. But few days ago, I found that light syncing server had stopped, so I made it run again. But after that it kept stopping so my backend server couldn't interact well with syncing server. I have no idea why this keeps stopping, and what should I do to keep syncing without failing?