Hi Dory,
It's extremely rare that Marketo would make concurrent attempts to access the same record or a record and its parent at the same time, so I don't belive that this could be a result of something changing on the Marketo side. If you have seen performance degaradations in SFDC it's possible that slower database transactions would make this exception more common, but it's really a question that I would take to Salesforce, as we don't really have the level of visibility from the Marketo side to make conclusive judgments regarding the cause.