2001 Vauxhall Astra mis-firing and losing power

I've been having this problem for nearly 2 years now! My car constantly mis-fires and loses power, especially going uphill. The engine management light comes on and sometimes even flashes. When i pull over, the engine cuts out. I've had it diagnosed a number of times and have so far had 3 coil packs, 2 lots of spark plugs and a treatment for sticking valves. The problem is still there and I really want some answers if anyone can help!! Thanks