astra mk4 engine fault

i have a 2000 mk4 astra 1.7 dti with isuzu engine, when driving on motor way at 70 to 80 mph after a while at this speed the engine noise changes to a much deeper noise and a slight loss in power and sometimes the service light comes on. i've serviced engine ( oil, filter etc) been looking on google and it said the cam shaft sensor oe edu unit.
anyone had same problem or can give advice

thanks