how can i fix my engine its loosing power at top end of rev range mk3 golf ?

hi engine runs well and pulls well with lots of classic 8v grunt but as soon as engine hits 3000rpm it starts missing and sounds very throaty and goes like its pulling a caravan does not smoke or use oil but eats petrol like its going out of fashon this problem happened in an instent not over a mater of time mk3 golf gti 8v