Why is my conection s o terrible?

i hav a linskeys cisco router and some rca modem, and i hav comcast. but my conection used to be perfect last summer, there culd 5 things usin internet and it wuld be perfect now its just 3 things and its just blown out. now my nat type is always moderate its just F'N terrible. so wat do i do? can routers go bad or somethin i heard my mom ask my dad that but he didnt no