how can i fix my mtd rs125/96 12.5hp lawn tractor?

i have a mtd rs125/96 12.5hp tractor lawn mower. it starts ok but when i use the clutch/brake pedal it wont move forwards or backwards. then all of a sudden it starts to work after pressing the brake/clutch peddal a load of times. then it will stop again. if i put it on full speed (no-6) and let the clutch out very fast thats when it may go again ?.