Hi Nora,
I checked the error log for your instance.. Amber's guess was correct you have hit you salesforce.com API limit a number of times over the past few days.
The SFDC API limit is a rolling 24 limit meaning that if use 100 API call now at 5pm I should get those 100 calls added back to my limit at 5pm the next day, this is why it might seem like the error is coming and going.
When the sync fails due to insufficient API calls the sync picks up where is left off once the API calls are available again meaning that the background sync should not skip any records.
For more info:
https://community.marketo.com/MarketoArticle?id=kA050000000Kyr6CACRgs,
Cathal