Are you running multiple Crystal Processing services as cluster in Production? If so, there could be machine which host Crystal Service not able to communicate to the CMS Service properly. It may be due to network, firewall, OS issue. Check with your admin.
Thanks Karthicse, I found the problem that was plaguing me. The administrator before me left Trace running on Crystal servers thus allowing the drive to become full on one of the machines.
I removed the trace then cleaned up the drive and the reports have been running fine ever since.