Engine revs while idlying

Usually when the idle raises at the rate you are mentioned, the AC compressor is cycling on and off, which is normal. Remember the AC compressor is on in defrost mode as well as when AC is selected. Turn the climate control completely off and see if you have the same surging idle, if not, then you know it is normal operation. You may want to have the AC system performance checked, because if the AC is cycling every 10-15 seconds, then the system is probably low and may have a leak. Good luck.