sounds like it wants to back fire

i have a 89 cavalier 2 dr coupe. an when i an driving about 60 then start to slow down without using the breaks it sounds like it is kinda back firing what could be the problem?? another promblem is when its either in park or an of the drives (1,2,d) it vibrates real hard until i start to go.. been told it could be motor mounts but its original motor with only 42k miles on it all together.. an last of all what can i do to get it to take off faster it starts up slow then gets to about 25 then takes off....... thanks, Tony