Intermittent delay on business rule

All,

I'm experiencing something very strange in a new install of 11.1.2.4. Users can save a form which runs a business rule which takes approx 10 seconds. However every so often a user will save the form and it takes about a minute to save (while nothing else is running).

I did some analysis, it seems that the Essbase calc is finishing within 10 seconds, but in Planning the "saving" wheel keeps spinning for another 50 seconds while Essbase does nothing. Finally it then comes back, and job console says that the business rule took the full minute (note that this is Calculation manager).