1991 Honda Accord erratic idle

I live in Madison, WI, and recently (when my problem started) it has been consistently below freezing. My problem is that when I start my car, it will idle higher than usual (about 2000rpm, as opposed to normal of 750-1000rpm). It does this every time I start (regardless of time of year), but the idle returns to normal as soon as the car warms up. However, my car has recently started to return to normal idle, but then rev up to 2800-3000 rpm and then return to normal, then rev up again, so that when I'm driving in third gear at about 33 mph and 2200 rpms and take my foot off the gas, the car will jerk like it wants to go because of this erratic revup/back down idle.
Any help would be appreciated.