A4 2.4 sport 2003(52plate) 50K miles

Occasionally the multitronic gearbox does not engage for approx 10 seconds on selecting drive or reverse. This seems only to happen when the car is driven from cold for about 5 minutes. The engine does not respond even if the throttle is pressed fully. This is an intermittent problem for the last 2 years and Audi cannot find the problem because it doesn't do it when they have the car. I have asked Audi to record my problem as it started while under warrenty. Any ideas?
Gearbox transmission oil has been replaced at 40K miles but no change in fault