97 ETC - Low Tach and Speedo Reading

This is my son's car, which we've only had about 2 weeks, so we don't know its history. It's speedo has been reading 10-15 mph lower than reality since he got the car. I still have to get someone to follow behind me to verify, but after 35 years of driving you get to know when you're doing 35 versus a 20 reading on the speedo. So I'm looking for ideas on where I might start looking for that problem.

The other problem is related to RPMs. The other day, we started the car, it idled fine for a couple minutes while we jockeyed in the parking lot, then as we put it in gear and started to pull away the engine revs climbed all by itself to about 2000 RPM. I had to slam on the brakes and throw it into Neutral. The car continued to "idle" at about 2000 RPM until I quickly goosed the throttle, which made its idle drop. Later, I checked for a sticking throttle (none) and anything else that looked out of place... nothing. There's lots of new sensors in the engine area, including the throttle-related sensors (maybe previous owner trying to nail the same problem), and they were all hooked up and still gleaming like new.

The computer threw a "High Idle - Current" code (I forget the number offhand). Now, the tach seems to read only about 300 RPM at idle. I don't have a hand-held tach to verify, but the engine seems to be idling at a fairly normal rpm... maybe not 750 rpm, but I'm pretty certain it's higher than 300. It's possible that the computer-controlled Northstar really can idle very smoothly at 300 rpm, which is why I need to verify the tach with a hand-held.

It does have a bad A/C Compressor-Clutch (get Low A/C Charge warnings) which is getting repaired tomorrow, but I am assuming it is unrelated to the above problems. The A/C is the only "known" problem with the car.

So I'm at a loss about the Speedo and Idle problems, and I hope someone could point me toward some troubleshooting ideas. A search on these forums didn't reveal anything.

Thanks.