Picking up speed - Engine revs?

1992 Pontiac Grand Prix mileage: 80,000. I purchased this car about four months ago. Ever since I bought it, it has had this problem: after starting the car, it revs at about 1800 rpms for about ten seconds, then jumps to anywhere between 2500 and the rev limiter at 3000 rpms. It stays there until the engine is completely warmed up, and since I am in Daytona Beach, it is about seven to ten full minutes. Also, when it first jumps up, the check engine soon light comes on. It only happens first thing in the morning and at about four hour intervals thereafter; has to be cold. I took it to my mechanic the next weekend. He said the battery was only reading 11.45 V and should be replaced. I will soon. He also said that it could be the air conditioning pressure valve, sensor, or something, as this was in the same circuit as the idle air control. He also said that a sensor in the air cleaner box was reading 40 degrees below zero. (I thought that might have some effect on a cold start) but he said after he cleaned the contacts, all read properly. The next morning, same thing! I am getting frustrated here, as I know that this can't be good for this engine. I am considering going to the dealer for this, but I have had no dealings with them and am skeptical about their efficiency and honesty. Can you give me any advice? I am somewhat mechanically inclined and have a rudimentary set of tools. I am reluctant to mess with an exotic modern engine such as this one though. Please forgive my long explanation, but I am hoping that the more information you get the easier the diagnosis. Also, if this helps, I have an oil leak coming from about half way up the block. The whole bottom of the engine is soaked. It seems to have gotten worse after a recent four hundred mile trip. Maybe the problems are related. Any advice would be appreciated.

The "check engine" light will "light" when there is a malfunction in the engine management system. The fault code is then stored in the computer. The computer must be scanned with a scanner to retrieve these stored faults to pinpoint the malfunctioning component.