NASTY99VETTE

Hey i have a 99 corvette ls1 coupe..ok and im having a problem with it. ok it started about a month ago. at first the motor would shake around about 70mph, like it was kinda misfiring or something. and then it stops for a day or so. ok its been a month since it started and now it does it at low speed and high speed. but it comes and goes, sometimes it will do it and sometimes it wont..so i changed the plugs and wires and it still does it just as bad if not sometimes worst. does anyone know would it could be or what i can do to fix it.