Mine was doing that, I have a 2006 GT.
I had to replace the Alternator to get ride of the surge while driving and cleaned the IAC really well for the surges to stop while at idle.

Note:
I was getting the Cam alignment error due to a faulty diode on my Alternator.
A few shops said if the Alternator is going bad it will produce surges at idle and while driving.
Dame thing cost 300.00 smackers for a rebuilt one with lifetime warranty.
Funny thing is the only place I could find one fast was Pep Boys.