my astra did pretty much the same thing after lots of self checks i had to refer to a mobile diagnostic expert which cost 30quid to find my knock sensor was at fault (which is why light comes on around the 70mph mark) this had gone wrong because the resistors in my alternator had gone, unfortunatley this means replacement of sensor at 48quid and alternator at 79 recond, my advice would be get a sensor from scrap yard just to try it before commiting to pricey parts
joflo, October 2008
