Hiya,
miner stopped 2 times in 36h with no option than hard restart of Apollo. Miner restart nor reboot system in GUI make miner go back to work.
Any indication in syslog about miner failure?
The syslog is gone if you end up needing to power cycle the Apollo in order to get it rebooted or responding again. The logs are stored in ram and only synced back to sd card if the shutdown is graceful. If the error situation is caused by something the kernel sees then some information is likely to end up in the log. However, thermal information isn't being logged.