Yesterday I did an oil change and replaced the air filter. I ran it for a few minutes to check for leaks, then took it on a short drive. Everything seemed fine until I got about 10 minutes into that drive. I noticed that when taking my foot off the gas pedal I was still gaining speed (I had to constantly be on my brakes). After getting back and turning the car off/on again, the car initially idles to between 2000 and 2500 rpm, then drops down to 1500, and surges to about 1800 every so often. It won't drop any lower than 1500. Idle speed was about 700-800 rpm before, and I never had any issues. I can't seem to find any unplugged connections or loose hosing, so where should I start? What am I missing? Thanks.