Please help, any advice would be greatly appreciated, could it be a dirty MAF sensor or time to change the spark plugs / ignition leads?

Over the past few weeks the idle speed has gone from around 600/700 rpm to just under 1000rpm.
When I stop and the engine idles, it often goes down to around 600/700 rpm, tick over is almost smooth, but then the idle speed slowly increases until tick over is perfectly smooth.
If I hold it on the brake and put it into drive then the engine is loaded and the revs go down to 600 and it runs perfectly smooth.
Normal driving and performance is fine.