2004 Vauxhall Astra loss of power

Wondering if anyone could help. I have a 1.6 Astra sxi. Whilst driving along I had a sudden loss of power and the engine management light started flashing. After switching the ignition off and back on the problem seemed to disappear although management light stays on permanantly now. I took it for a code read which said the fuel tank was empty although tank was half full. I was told that it could be a dodby batch of supermarket fuel (possibly had some water mixed in with it and best thing was to put a bottle of dry fuel in the tank). Problem solved i thought as it's ran fine since although management light is still on as I haven't had it deleted yet. A couple of days ago it lost power again all of a sudden (I've not used supermarket fuel since). It seems to judder slightly when standing in traffic etc and sounds rough a bit as though it's stuttering when running. When the accelerator is down in 5th gear the management light flashed continuously until I take my foot off the peddle and then it goes back to staying on permanantly. Agian, if I switch the ignition off and back on, it'll run and accelerate fine for a few mins until the probelm sequence starts again. My first thought is plugs or ht leads but any advice would be appreciated.