1990 Other Volkswagen Models MISSFIRE

HI, HAVE A 1990 1.8 16V VOLKSWAGEN CORRADO, ON THE THROTTLE HOUSING THERE IS TWO CONTROLS, A FIRST STAGE FOR THE THROTTLE, THEN THIS OPENS THE SECOND STAGE OF THE THROTTLE, ON THE FIRST STAGE THE CAR RUNS FINE, WHEN IT GOES ONTO THE SECOND STAGE OF THE THROTTLE IT SEEMS TO GAIN A MISSFIRE OR JUST STARTS RUNNING REALLY ROUGH, ANY IDEA WHAT THIS COULD BE PLEASE, COULD IT POSS BE LACK OF FUEL, MAYBE THE FUEL FILTER NEEDS CHANHGING, DONT KNOW, RUN OUT OF IDEAS AND THINGS TO TRY SO ASKING FOR HELP PLEASE, MANY THANKS