2001 Vauxhall Astra intermittent eml light fault

My daughters car has been acting up with an intermittent fault. The car runs ok round the town and on carriageways around the town.
It idles ok.
Once on the motorway after about 15 to 20 min’s at around 65 - 75 mph the EML light comes on and the engine goes into limp mode. If she pulls over onto the hard shoulder and switches the ignition off and then back on again it starts ok with the EML light out and then after about 5 to 10 more min’s driving it happens again. If she pulls off the motorway resets the EML light by stopping and switching off the ignition and back on again she can then using back roads drive back home without any problems.
Garage said it was the EGR valve sticking and removed it and cleaned it, she tested the car next day and after driving around the town for 20 min’s without any problems she tried it on the motorway, after 15 min’s on the motorway at around 65 - 75 mph it did the same thing again. A friend of hers after looking up faults on the EGR valve on the internet tried blanking off the ports on the EGR valve with a blank made out of a coke can and they tried it again but it did the same thing again. Took it to another Garage for a fault test with a plug in diagnostic tool they said that no fault codes were showing up.


Any ideas on where to start would be appreciated, its been years since I have been under the hood of a car