I've got a problem with the idle on my 96 LS400, 145k miles. The car is up to date on its servicing and there are no warning lights on. The car starts fine, hot or cold, and drives normally without complaint under any driving conditions, except when the problem occurs. After a run of about an hour or more at motorway speeds, when the car is coming off the m-way and slowing down at the end of the slip road, engine revs will occaisonally drop so low that the engine cuts out. It will re-start from neutral without any problem and will not repeat this until it's on another long run. So, it won't cut out again when I stop at the next junction or set of lights. It only seems to be a problem slowing down after the m-way run. On a regualr trip I do, I've tried running with the cruise control off (normally it's on on m-ways) but this makes no difference, the problem remains.
I don't know if this is simply a problem with the idle speed adjustment - it ticks over at at about 400 RPM which I think is a little low - or somethink linked to idel air control or temp sensors.
Any pointers would be welcome.