Wow, this a blast from the past! Don't think I've ever had a reply to a post as old as 8 years before!
Thanks for the response, but this has long since been resolved lol.
IIRC, it wasn't my query that was causing the problem, SQL Agent was reporting never-ending jobs, meaning the next scheduled run could not begin. Further investigation prompted this post, so my query was only showing what the SQL Agent itself was using.
After much reading and testing, the decision was made to delete/modify certain job history/activity records, which in itself didn't resolve the issue, but then following it with an agent restart, everything ended up working again. Restarting the Agent by itself did not work, records did have to be manually adjusted.
BTW, your blanket advice to never adjust system objects is great for newcomers to SQL, but glitches and bugs do happen, and sometimes you need to get your hands dirty.