I have a masternode that keeps disappearing from dash ninja

The 5 min port check on 9999 shows no probs. I restart the service, kick it off with the wallet and it returns to dash ninja for an hour or so, then drops off again. I can only presume it's a ninja issue, because the port check never has any downtime. I'll just keep my fingers crossed for a week or so (and stop pushing it to the back of the queue by starting with the wallet)
Any ideas appreciated though!