Audi TT - intermittent starting issue

• When it occurs, the engine cranks 3-4 times, fires but just at about 500-600rpm, and then immediately picks up and comes to rest at the normal idle range around 900rpm.
• ALL other times it starts within 1-2 cranks and jumps straight into life.
• Very difficult to predict when it will do it. Today it has done it after a short run from cold (3-4 miles), and then coming back after around 40-60 minutes, when still warm.
• If I return to the car within 10-15 minutes, it will guarantee to start fine.
• Isn't battery as it starts fine in the mornings.
• Isn't pump as the performance is spot on.
• Plugs are brand new BP7RE.
• Oil is brand new Mobil 5W/30.
• Replaced (and tested) water temp switch.
• Just cleaned out MAF, Throttle and AI sensor, and reset position.
• NO codes whatsoever thrown up.