Engine power loss

Hello there,

I have a Nissan Primera 2001 1.6cc and it starts fine but when accelerating there is loss of power. This is big time loss though. For instance if I am at 2500 revs and put my foot down it does nothing. I can hear some form of gaseous noises but cant work out what it is. Codes PO300 and PO171 came up some time ago, these are random cilynder missfiring and emmissions levels up. I have changed fuel filter, spark plugs but nothing to do with these as no difference was made. If I slowly acceleratte and reach say 4k revs then will not go further and the revs jump up and down at that point randomly. Fuel pump maybe is going or something is blocked in the fuel line which I wouldnt think thats the case. There is also a valve which controls the pressure as far as I know and could be that too. I would really appreciate your expertise opinion on that. From your experience what is it more likely to be and what costs are associated roughly. If need further info please let me know Tx I will donate if satisfied with answers