Horrible Shifting Problems

i have a 00 ford focus ZX3 with 145K Miles on it. It started to do this thing about a year ago where if i was in the wrong gear and trying to go up a hill or something it would just start stalling out and it would act like i was in the wrong gear. usually just stopping and starting again fixed this problem. but just yesterday i was driving and it starting just stalling out on me. i was in the right gear and everthing but it would putter out and the whole car just kept stalling out even when the clutch was in and everthing. i let it sit while i waited for my ride to come and get me and then the car was fine until i put it into forth gear and then it started all over again. Help! Does any body know what the problem could be?