iCal has started losing data

My wife and I have a shared diary, accessible from iMac (running Yosemite), our phones (iPhone 5) and iPad. Some time back Apple changed the views for iCal on the phone, so we use myCal as a front end on the phones. Over the past few weeks entries made on one device have started to take a very long time to sync (hours) but worse, data added in the 'notes' field has disappeared or reverted to an older version. I'm guessing that this is something to do with the behaviour of iCloud, and the loss of notes made on the phone is down to the entry being 'restored' from iCloud. Anyone got any thoughts about how to improve the performance?