Hi, ive had a similar experience. Car juddering from time to time - felt like it was misfiring and id only had the car 3 weeks. Fair to say I wasnt impressed. To add to the difficulty no warning light came on. Took it to lexus and I think the default response is erg valve. This they cleaned but as I went to leave the garage, the car started 'mis-firing'. They plugged it into diagnostic and the result was faulty injector. This has now been changed and the car is running fine, at the moment. Luckily I didnt have to pay the bill on this as it was around £600!
Ive read a lot on this forum and its a great source of information. As much as I love the toys on my new car, I am concerned about the reliability of the diesel engine. A lot of problems appear to come from carbon build up so ive ordered some bk 244 engine cleaner - some great reviews on this stuff so Im hoping this prevent further problems.
I hope you get your problem sorted.