RPMs surge to 3000

A few months ago this started. Worst surge ever. The engine RPM will go up and down randomly while driving or idling. When it's bad it goes up to 3000 rpms exactly and will stay there for about 20 seconds or so. If it happens when I'm driving I have to downshift a gear to keep from speeding. It seems like it's worse when it's raining or cold out, but really it happens any time. There were no CELs but the first thing I did was check for codes. None in CM, KOEO, KOER. I always heard that most surges were due to vacuum leaks so I bought a bunch of vacuum lines and replaced anything that wasn't nailed down. No difference. The car runs really well other than this problem. I hadn't done anything to the car besides drive it before the problem started. I've seen millions of threads on idle surge in all the forums. I don't remember seeing any that went up to 3000rpm. Car is mostly stock with some mild bolt-ons done a couple years ago and has no cruise control, which is kind of what it feels like. If it gets a little warmer this weekend I'll clean the IAC, although that is always recommended for an idle surge between 800-1200rpm. So the 2 things throwing me for a loop are 1: that it surges so high, and 2: that it only does it sometimes and completely randomly. I've also checked under the hood for any obvious things like disconnected lines and such. Any ideas? I'm open for suggestions before I take it to a shop. That can get expensive if it doesn't do it while it's there. It does it maybe 2 out of every 3 times I drive it. Thanks in advance.