Geth running on AWS lose peers after a couple of hours

Hi everyone,
I’m running a geth 5.5.1 to sync ETC mainnet on AWS EC2 instance. Every time after it started, it will find about 7 to 9 peers within 10 minutes. However, after couple of hours, the geth will lose most of the peers even all of them and then stop syncing.
I have opened all ports to public internet and tried to add static peer in geth javascript console. But it seems still bad.
What should I do now to have more peers connected geth performed uninterrupted?