engine management light

the engine management light on my dashboard keeps coming on everytime i reach about 70 mph ,it disappears if i slow down to about 60 mph.it is very sluggish when the light is on and is unable to accelerate.
the car is a vauxhall astra estate 1999 diesel.
any ideas what might be causing this , i have a roof box permanantly on my car and the first time i had this problem was the first time i had the roof box on,however it didn't appear again for months ,now it is happening all the time.

I have a 2001 Astra hatchback, petrol engine.

I had a similar problem to you a few months back: the engine management light came on. It turned out to be the Exhaust Gas Recirculation valve (part of the emissions control system).

I didn't notice any change in performance, but when the light came on I was obviously rather ginger with the silver machine so it could be that if I had floored it I would have found it sluggish.

Before I realised it was still in warranty, I researched the possibility of using engine diagnostic checks to find out the problem. With some engines, although this is becoming less common, you can bridge a couple of connections and then count the flashes of an LED. Later engines require a computer to be plugged in (a Tech-2 was used at my Vauxhall dealers, I believe).

It is worth doing some research on whether or not you can use diagnostics, perhaps in a Haynes manual, just in case it is something you can fix yourself.

My friend had a similar problem with a Peugeot 309GTI. The light would come on after about 15-20 minutes of run time. Peugeot diagnosed a faulty ECU and quoted £1200. I wondered about a dry joint being moved as the car warmed up. Anyway, we took the ECU out and examined the board with a magnifier. There was a cracked solder joint on the edge connector. Re-soldered the joint and all was well!

Now, your problem may be different, but if you get to the 'replace ECU' stage, it might be worth a look.

I had exactly the same prob with a 1987 Maestro!! Fixed with a 10p resistor.