Could be the EGR valve is clogged up. I had this a couple of times and not long after the "check vsc" warning and limp mode scenario would kick in. One time it killed the cat and would have been a £2,000 - £2,500 repair as the car was >1 year out of warranty, but Lexus covered it as goodwill gesture (it probably helped that the car was serviced by Lexus / Toyota though).
To avoid a repeat of this and any potential head gasket / DPF/ DMF failures I now drive a Toyota GT86 - miss the Mark Levinson, reversing camera and some of the refinement - but good fun and better on fuel. Will probably return to Lexus in a couple of years time once the new IS is bedded in and if they do bring out a couple version as hinted.