How do i make my wheel chair front wheels stop wobbbling/fluttering?

I have a Quickey RX wheelchair (i'm not handi-capped i just have it for fun) and i want to be able to go really fast down ramps and such in it. but as soon as i start going (usually about 7-8 MPH) my front wheels start wobbling and slow me down. is there any way to make those front wheels stop wobbling/fluttering so i can get going at a pretty high speed?