In SSRS 2008, we have some reports that are scheduled to run automatically. One of them has a subscription which runs at 5:00am every day. The other is executed from the last step of a sqlagent job. They both run as they are supposed to for months on end. Then they will sometimes not run. This lasts for a few days, then they start running properly again. Sometimes, stopping the SSRS service and starting it again helps but not always. They both embed the report into the email and are using MHTML. I think I have seen that subscriptions are more reliable when attaching the report as a PDF instead of embedding it, but that is not really an option with these.
So my question is simply, Why do they stop working for a few days then start again on their own. The server did not get rebooted at any point during troubleshooting this.