List currently executing pages or scheduled tasks

I wondered if there was a way of executing a scheduled tasks, lets say every five minutes. But it needs to check that its not already running. i.e. that the previous execution has not taken longer than 5 mins.

I know that you can do lots of clever things with gateways but they don't do things serially and the order of execution is absolutely vital in this case.

This might be a way round an issue I hit in a previous post. http://forums.adobe.com/thread/505742?tstart=0

I figured I could process them in blocks of 500 every 5 mins. And just exit out if the previous block hadn't finished yet.

Any pointers as to where I might look?

many thanks

Steve