either:

1) air in the lines - bleed them until you don't see air bubbles
2) master cylinder leaking - check around the clutch pedal for any stains or fluid on the pushrod
3) brake fluid is reaching boiling point (due to many contaminants) - flush and replace with OEM or better fluid
4) brake caliper pistons are wearing out, leaking - check that out (or replace calipers..not that expensive)

the only time you would experience brake fade is when the pads heat up past their critical operating temperature.