Mysterious Issues With '95 Firebird

I have a 1995 Firebird (3.4L V6) that I got a couple months ago that has a couple strange issues. It sometimes has trouble starting if I leave it sitting for more than a day, and it shakes and vibrates when I maintain speed around 45-55. I have had a new battery and new sparkplugs installed and it didn't seem to fix the former issue. Kind of wanted to get an idea of what might be the problem(s) with it.