i bought '98 R reg vectra estate. after couple of weeks engine management light coming on once 70-80mph reached on but went off once engine turned off. there is a strong smell of petrol f.rom the exhaust and now and again it cuttung out when clutch is dipped.doesnt sound like it firing properly when accelerator pressed and released the rev counter drops to almost zero causing the car to almost stall. i changed plugs and cam shaft sensor already. rocker gasket and hd leads replaced 2 months b4 purchase.any suggestions??