Hi, not sure if this has been answered but I had exactly the same problem in my 2007 IS220d like you I have the ML kit and thought it must be an amp problem. I had the amp swapped under warranty earlier this year due to a fault and was advised that it was this again. I took it to Lexus Coventry who replaced both amp and head unit in an attempt to resolve the issue but everytime the estero went above 40 it cut out. They advised me that it would require further investigation at £60 per half an hour, so I thought I would do it myself and the first thing I did was to send the sound into each corner, front right, back left etc. All worked ok and until I got to rear right and them it cut out. I removed the seats and rear shelf and disconnected the speaker and it worked fine. I then tested the offending speaker on the other side and it continued to trip the stereo. My advice check the speakers first, replacement part is £60 inc vat.
