My ls400 always cuts out as I pull into the services after several miles of motorway driving at speeds of say 80 to 90 mph
I was told to fit a new petrol filter but it has made absolutely no difference whatsoever, As soon as I slow down to 4 or 5 mph on comes the ignition lights & I have to restart the engine. Engine starts first time & runs lovely. No more problems untill I come off the motorway again. It does not happen if I keep the speed to around 65mph. but if I have been travelling at around 80 or more even if it is only a mile or so then it will happen everytime. Why? Any advice would be appreciated

Start with cleaning the throttle body. Check the throttle position sensor is adjusted correctly.

Yep throttle body clean. Temporary fix - turn on the A/c when you pull of the motoroway, extra revs keep the engine alive. Very common fault, have a look at lexls.com for the tutorial on cleaning the body.

Sounds like the infamous boot hinge wire harness issue.
Classic symptom.
Have you checked the wire harness?
The ECU is getting bad data from the short there, causing the cut off, is my guess.
Post back if this turns out to be the cause.

Good luck.

Thanks Guys. Yes It was the throttle head that needed cleaning. After doing this little job I came off the M2 last night & it was perfect. Engine is running lovely now. I am very happy so a "Big Thank" You to you all

Thanks for posting the solution, glad it's fixed