Hi Philip,
You can find the information by configuring the audit.Audit will write all the information regarding the no of records, Time to perform the report refreshing the report process.
when ever any heavy report runs or schedules, Data will be stored in the Audit DB.
Later you can compare the issue occur time and report run time to know the report which is causing the issue.
Below is document to configure audit.
SAP BusinessObjects 4.0 Auditor Configuration & Deployment End to End