Logging of diag output

Is there a 'good' way i can log diag sys top-summary output other than logging from a console session?

What does the --d option mean by 'dump stats to files'?

I've got an occasional problem with CPU hitting 100% on a 61E that's in an isolated network with no means of active monitoring and i want to pin down the process, i suspect it's ipsengine as it tends to happen during heavy file transfers, though not always. 

Thanks for any tips