Vauxhall Astra loss of power and rough sounding engine

My astra will start and run fine for the first 3 or 4 miles then lossing power. Getting up hills takes forever and it will accelerate on the flat but a lot more slowly than normal. On low gears it sounds very rough as if it's making a blowing sound and if I rev it when standing it hesitates before actually revving the engine. Also when it does start to play up the engine management light will flash every time I go over 32mph. Take my foot off the pedal and it'll stay on but not flash. Strange thing is if I switch the iginition off and wait a minute or so and start it up it's fine again. But after a couple if miles the problem will be back. I've had a code read which said P300 (random cylinder misfire) P301, P302, P303 and P304 which is all 4 cylinders were random misfiring and P1300 (low fuel level) even though there's plenty of fuel in it. The fuel filter is a lifelime one fitted inside the tank and I've replaced all 4 plugs and even the coil pack within the last week without any luck. Any ideas?