Engine surges

The engine idle speed is controlled by an Idle Air Control valve (IAC). When they are bad or going bad, idle usually lopes up and down, so that is my first thought on that. It can be confirmed if you have a live data scanner which you can watch how the valve reacts. At highway speeds, the IAC does nothing. Air is then allowed to enter the engine through the throttle body. If you are getting a surge then, I would suspect a fuel filter, weak pump. This would require that you check fuel pump pressure. Watch this video:

https://www.youtube.com/watch?v=peh7-pFycyc

If the check engine light has come on, have the computer scanned for trouble codes. If they exist, it will indicate where the problem is coming from. Most parts stores will scan for free.