1997 Other Volkswagen Models Acceleration

Engine Mechanical problem
1997 Other Volkswagen Models Two Wheel Drive Manual 72000 miles

Hi, im from England, but I couldnt find another site like this so I thought id ask you guys, ive got a 1997 reg VW polo 1.6 litre, the problem is, when I floor my accelerator, the car revs incredibly and 'redlines' it without any kind of forward motion, the car just rolls. It feels like its popping out of gear, but its not. It usually happens at times of hard acceleration for example onto a motorway in third or fourth gear. But also the car revs really too much in first and fith aswel. But it doesnt happen if I only hold the accelerator half way down and dont put too much pressure on it, if I keep it like that, its fine, its if I put the pedal to the floor it will just rev! Im not sure wether its my gearbox, clutch, accelerator? I have no idea, please help!

cheers, charlie