Is the Cleanup Task Data "Maximum number of working database rows" set to 1,000,000?
You may need to do a one-off cleanup if the task isn't ever catching up with itself:
"Taskinstances table growing rapidly"
http://www.symantec.com/docs/TECH144662
And:
"Can't see task details after a failed run, "This task run has been deleted and cannot be vieweded or re-run. Only summary data exists - create new task"
http://www.symantec.com/docs/TECH180203