Hello Dale,
I have seen this in the past due to wrong sequence numbers in the tasks table (database level). The way to fix it is usually, as you mention, to do a roundtrip using MSP or to correct the sequences at DB level (do not try without Support's involvement). If you switch to OWB, OWB may also fix the problem but if it does not, switching to MSP would still be an option, but mixing schedulers is generally not recommended as each one has different scheduling criteria and could lead to some undersired changes if unnoticed.
As you mention that this is a really rare event, if you are using OWB and OWB round-trip does not resolve the issue, I would recommend you to open a Support case.