My car is coming to the end of it's 3 year warranty and there appears to be a problem that only occurs very intermittently. There can be weeks before it happens again. After the car has driven about 4/5 miles from cold, when accelerating from lights or out of a roundabout there is a thud from under the car accompanied by a hesitation/jerk, followed a second later by another and then another. As his happens when in 2 wheel drive I suspect the auto gearbox. I have taken it to the main dealer but it performs without any problem when they road test it. Any ideas on what is the possible cause as I a concerned that the warranty is ticking away.