I am having the same issue. with Orion 9.1 SP5 with a couple of addtions. The SolarWinds Alerting Engine and NetFlow service fail as well.
It seems that they fail about once a week and i see nothing in the event logs about why they fail.
I am going to open a ticket about it.
If you have any advice for me please let me know.
Thanks in advance.
I've not yet fully resolved this issue but as a result of the ticket I opened they had to perform a series of uninstall/reinstalls. This has helped reduce the number of times the NPM service fails from several times daily to several times weekly. I'm running a single poller environment and currently in the process of reducing the number of monitored interfaces. It seems that we've exceeded the the sizing recommendations and this may be the cause for the continued instances of instability.
I hope this helps.