I am assuming that you haven't had a chance to get your vx scanned yet. I am a firm believer in changing all fluids when buying a used vehicle.
Our cel would come on and off when I first bought it. I scanned it, came up with a code for the IAC, replaced and no problem since. Autozone will do for free, or sell you a std scanner for $40 (I think).
Post the code when you get it