"On-access scanning is disabled"

In the last few weeks, the error message "On-access Scanning failed to start" has spontaneously cropped up a few times; when clicking on the S in the status bar, the greyed-out top line reads "On-access scanning is disabled" instead of "Up to date".

Removing and re-installing has corrected the problem—but only temporarily. At some point, the error occurs again. It's hard to complain too much, seeing as the software is free, but it's getting to be unreliable.

I'm using the latest version on a MacBook Pro with OS X Mountain Lion 10.8.2.

Has anybody figured out a more permanent solution to this problem?

Thanks,

Kevin