Late Calendar notifications
Over the last few months Calendar has become very unreliable at notifying me of events. Notifications appear on my desktop but they are always late, anything between 10 minutes and several hours - there doesn't seem to be any pattern to it and it varies from day to day.
Also, I might have three different events, set for 6 am, 7 am and 8 am say, but the notifications will all appear at the same time at some random time after that, so it is not as if everything arrives two hours late, say.
I have quit and restarted Calendar (many times), and have experimented with new events and changing existing events, but nothing improves its reliability.
I am using Sierra 10.12.6 on a 2013 iMac.
Can anyone help please?
iMac 27", macOS 10.12