My first suspicion is that there's a big 'ol whopper of a node in the queue that's putting you over the limit. There are a few ways to test for this:
Check the search_dataset table in your database for the largest sid that corresponds to a node one your site. Then, look at the nodes with larger nids than that. Is one of them abnormally large, or otherwise weird? Try editing it to have a blank body (be sure to keep the old revision) and see if that helps.
If not -- comment and we'll try something else.
