Why does my 2005 Hyundai Accent surge while I'm driving?

[mistyd_roberts]

My Accent, which has 53,000 miles on it, almost always surges when I am driving between 40 and 45 or 60 and 65 mph. This has been happening since November 2008. At first I thought it might be water in my gas tank, but now I'm pretty sure it's something more serious.