Lexus say it does not happen but want £1100 to look, Bosch

Lexus say it does not happen but want £1100 to look, Bosch say it does not either overheating GS300. Any ideas?
Any time I drive my car over ten miles it cooks up. There was no "event" no bang, nothing, it just started to run hot. Lexus changed the thermostat for £128. and it was okay on short runs. I drove 350 miles up the M1, good. Coming back, just straight up into red. Now Lexus said it did not happen when they had the car but wanted £1100 to do the cylinder head gasket in case, as no oil or water was cross contaminating, I thought I'd attempt elsewhere. Bosch centre could not get it to overheat either, but said the liquid coupling fan wasn't running. Next time I took it 15miles it cooked up. I drove it back at the top of the red and showed them. Having been switched off for some mins before restarting, the fan runned, temperature plummeting. As they were at a loss, nothing happened since. The car is in limbo can not fix it, can not use it, can not sell it except at big loss . Does anybody Know how to fix this or where to start?

are you saying that the fan does not work occassionally?
have you checked the power supply to the coupling fan, and earth to see if cables are broken transit may loosen a connection.. sorry can not be more particular. lexus not my model.
good luck.

Scrap it.

WOW. Bosch really is bad at electronics. So far I have heard nothing, but bad things. I had my VW Passat's ABS Bosch made module malfunction. And so did many others, it is a really a factory error, they made a whole line of cars like that. With flawed electrical boards. I could check up on this and see if you may be able to find others like you, or in alike position. attempt websites that are run by Lexus fans, forum boards, etc. i.e. like this VW one, vwvortex.com, so you have an idea. I learned what was wrong with my module on this kind of forums, then also learned how to fix it myself for an very low price. If this is an frequently occurring problem there will be 3rd party businesses that may help you out. Look into it. it will end up being much cheaper than dealing with the stealer ships, and you will most likely still get at least a three to five yr. warranty, depending who you deal with.

Sounds like a flawed temperature sensor, easy to change.