$session->scrape() returning control too soon

I use a simple test script that launches a scrape session against a running server. The script connects fine and the scrae session begins.

In a number of cases, the $session->scrape() function returns control to the calling php script before the scraping session has completed.

The scripts work fine within the Workbench. There aren't any error logs.

These are long-running scrapes with large amounts of data.

I've increased the server timeouts and memory, without any effect.

Anyone have any ideas on how to instrument or debug this?

Regards,
Fred Dinkler