This just started yesterday on my 3.4.4 forum (upgraded on May 10th and it's been fine).
When the " Daily Topic & Forum Digest " scheduled task runs. It completely hangs the CPU. If it's run as normal during a page load or from within the AdminCP then it takes the httpd process to 100%. If I change it so that it runs as a cron or manually from the command line then it hangs php in the same way.
I'm scratching my head working out what exactly is causing it. Anyone have a clue how to troubleshoot this one? I'm wondering if it's a bad email address or something but I can't see what is happening at the time it hangs.
Edit: Also, the "Weekly Topic" task seems to work fine still.