Idle / Loss of power issue

I have a '99 318 tourer and currently have an issue with the idle and loss of power. The issue started yesterday. It is an intermitant problem as sometimes it doesnt occur until I put my foot down a little. The idle will be anywhere between 200 and 1000 rpm, sometimes it recovers, other times it kills the engine. I have also noticed that when generally driving it doesnt seem as responsive as it usually does. I have changed the spark plugs to no avail, any ideas?