[development] cron woes - drupal_cron_cleanup() kills my cronjob while it's still running...

I am working a module that makes use of cron to fetch large amounts of 3rd party data, and then turns that data into nodes and sticks it all into the drupal db. Each time the module goes and fetches this data takes a large amount of time, and my module invokes a number of different scripts to help handle the data depending on where it's coming from. I'm running into problems where in the midst of my cronjob executing, drupal_cron_cleanup() seems to get called somehow - the error 'Cron run exceeded the time limit and was aborted' appears in watchdog. Does anyone have any idea what's going on and/or how I can prevent this? Thanks so much, Arthur