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!
I'm not sure if this is related, but I just noticed an anomaly in the Balance reporting at DashNinja for a fairly new MN. The node has received 4 payments. They all show up fine in the wallet, but DashNinja shows only the most recent one when it reports the balance.
Edit: Update - just noticed DashNinja also correctly shows the "Total Paid" amount for that MN. Only the Balance seems to be incorrect.
Edit: Update again - on further digging, I think it's probably a red herring - a side-effect of some Darksend Mixing. Sorry!