My Opel Astra Loses Power when driving, this usually happens after driving some miles like 30 miles?

My Opel Astra Loses Power when driving, this usually happens after driving some miles like 30 miles; it reflect an engine light on the dash board. If you do not slow down it maintains the speed but when you slow down it takes long to pick and this does happen when the rev/s is too high sometimes goes to 3 when just traveling at 40km/h. If you stop the car steering is too hard/locked. The engine light wil only clear wehn you stop the car and press (rev) the acellarator for 2-3 times. What could be teh cause? Hav etried for computerised diagnosis but they have failed to locate problem