I own a 2000 ford focus LX, recently the car began chuging....?

[Cinerd]

I own a 2000 ford focus LX, recently the car began to chug and have trouble pulling itself during excelleration (on occasion other times it ran fine). Through out the course of a couple months it got wrose and bega to do this all the time, kind of like it was missing a cylinder and began setting the engine light off. It stayed that way for a period of time until today it finally came to a point where it wouldn't hardly pull itself and it stranded me. It excepts full throtle but it won't pull up until that point and it will not keep idle without dying. Spark plugs where recently changed but it didn't do much to help it. Any ideas?? Any information would be greatly appreciated.... TY!