wp-cron – consolidate runs? Prevent unscheduled runs?

While dealing with server slowdowns, I’m getting into wp-cron and what it is doing behind the scenes. There are reports of it using a lot of CPU (which happened to me a couple of days back and brought the server to it knees). Installing wp-crontrol, I find that there are a number of cronjobs that really could be run only by the main blog. Each sub-blog has cronjobs that check wp_update_plugins, wp_update_themes and wp_version_check, for instance. These only need to be run once by the main blog; not by each and every sub-blog in the network; they’re redundant.

Is there a way to do this?

On another note, logs show that there is strong correlation between FeedBurner GETs and spurious wp-cron runs that immediately follow. There are 5 blogs with 13 scheduled wp-cron runs each – totaling 65 scheduled runs for a 24 hour period. Yet, wp-cron ran 305 times the other day; most of those preceded by a FeedBurner GET! No posts were added or updated during this period and none were set for posting at a later date.

What’s going on, here, and is there a way to limit this or set RSS-related stuff up in a way such that this doesn’t happen; if indeed Feedburner GETs are a cause?