1991 1.8 renault laguna power problem

I have a 1991 renault laguna its done just over 100,000 miles, i have upgraded the engine... like a perfomance air filter, ive put a turbo on it. I have been expieriencing a big power loss after about 3000 rpm and it starts to splutter almost like its hitting full rpm and struggles to go any faster so i have to change up a gear i have tried putting a new standard fuel pump in and a new standard fuel filter but its still doing the same thing please help