Tachometer on my car isnt functioning correctly

I have a Hynundai XG 350 and the check engine light just recently came on last week but nothing was going on with the car but now I'm noticing that the tachometer doesn't move when I drive but when I'm sitting still at a light its jumping up and down, and today when I started driving I could tell something was up when I pressed the gas it just felt different when driving then it was ok again, Is this enough info to tell whats wrong with my car?