Hi guys - I have been following this thread for some time now as I also have the same issue with my IS300 Sportcross. I bought the car from a mate of mine as it was his wife's ride but she hardly ever used it as she would drive their Range Rover and my mates a builder, so he hardly used either of the cars as his daily ride is his van. It was one owner from new, full history and had only done 12,500 miles when I bought it in Feb 2011 - so was virtually as brand new when I got hold of her.
Since then I have had 3 sets of front discs and pads replaced - everything was ok for approx the first 20k miles when I first bought the car off my mate, then the dreaded juddering started. I took it to an independent Lexus specialist (fully Lexus Trained and the right equipment for working on them). I had sort of diagnosed it as warped discs myself having had experience of this on my Vectra V6 SRI which used to chew front discs up after about 30k+ miles (I stuck 180k miles on the Vectra before selling her on, so had gone through quite a few discs by then, it was never anything else). Anyway my mechanic also concluded the front discs on my Lexus were warped and on inspection this was the case, so they installed new OEM parts to both corners.
Less than 7 months and approx. 15k miles later the same issue returned - my mechanic no questions asked replaced the discs and pads under warranty and at the same time comprehensively checked the front end, suspension, bearings etc for other issues but found nothing un-towards. However you've guessed it, the new ones lasted less than 7k miles this time before the juddering started again, so I took it back to my man again who was at a total loss and said he would take it to the main Lexus dealer who supplied him the parts for them to look at it, which he did. After waiting 2 hours for them to actually do a road test and some cursory checks with it on the ramp and expecting them to do further checks with a dial gauge for run out etc, which they didn't, they concluded the issue was at the back end and said he would need to bring the car back again for them to look at it closer!!
However neither my mechanic or I could afford the time to go back to Lexus again for them to investigate further and quite frankly we both thought they were off their heads to suggest the issue was at the back end, plus I couldn't afford to have the car off the road as I needed it for work and public transport was not an option for me as it involved a 12 mile drive to the rail station and 3 trains to get to the office. So we went with another set of discs, new pads and my man also checked out the front end again but still found nothing amiss. My mechanic supplied the parts FOC and just charged me a nominal labour rate to fit this new set.
Again everything was fine for a while but the issue eventually returned, however I have to date not had anything else done and have just lived with it. The juddering is mostly pronounced when braking in the 55> to 40 zone, but can also occur intermittently at higher speeds and be quite aggressive. What I have noticed is that when the discs are cold braking is fine at any speed and the juddering doesn't manifest itself at all - I have specifically tested this many times to make sure it is not a figment of my imagination - however once they have had the chance to heat up that's when the juddering happens. I have also observed what I feel is excessive amounts of brake dust accumulating on the front wheels??
Bizarrely though, as Doody also mentioned in his post, as time has gone by it seems to be sorting itself out - I have just got back from a holiday in Cornwall driving the car under all sorts of road conditions, at high speeds up and down steep hills and around sharp bends and it was on the whole very well behaved. Not being a DIY'er mechanic myself I have decided to wait until the car is next due a major service and at that point to get fitted some drilled and grooved performance discs from MTEC Brakes - unless anyone has any feedback that would advise against fitting them.
There is a very interesting and informative page on the MTEC website which I would if I could provide a link to here, but I can't seem to be able to Grrr - however if you go to the Mtec website and on the LH side under the articles section click on the link to "Brake Disc fitting Guide" what they say there regarding hints and tips to avoid warped discs seems very plausible and correlates to what I feel is the issues I have with my car. I used to take the first 100miles or so after having new parts fitted , carefully but as I was doing 110miles a day mostly on the motorway it's not always easy to look after them until they fully bed in, which MTEC recommend should be 200miles min.
I will post again when / if I have anything further to add. BTW just sailed through another MOT and apart from this juddering issue I have not had any other problems with my car - she is just coming up to 100K - but now I don't work in the same office and am using a train to get to work I don't anticipate putting more than 6k miles per year on her going forwards.