Try using Analysis Workspace: It has most dashboard reporting capabilities, and has much better performance.
Split the dashboard up so it has less reportlets: I have seen intermittent issues when a large amount of reportlets attempt to request data simultaneously.
Schedule the dashboard daily via email: If you only look at the dashboard daily, send it to yourself via email since you mentioned the PDF's are working correctly.
Try using Analysis Workspace: It has most dashboard reporting capabilities, and has much better performance.
Split the dashboard up so it has less reportlets: I have seen intermittent issues when a large amount of reportlets attempt to request data simultaneously.
Schedule the dashboard daily via email: If you only look at the dashboard daily, send it to yourself via email since you mentioned the PDF's are working correctly.