I'm sorry about your troubles. Luckily I may be of some help.
The GTO I took out for a test drive had the same problem. The tach was bouncing all around, the speedo was indicating anywhere from 60mph-120mph on a road where I was travelling 40mph at the fastest. I didn't pay much attention to anything else which was or was not working. I was very lucky that the salesman was in the car with me at the time, and he noticed the same problem with the electronics. He had me pull over, shut the car off, and restart to get on our way back to the dealership.
After the test drive, we pulled the car into the service bay, and it was back on the lot the next day. Apparently, there is a short somewhere around the glovebox area. I believe they were saying something about the ground for some of the wires was funny, so they will fix it free of charge.
If it was me, I'd take the car to the dealership whenever possible, and have mention the problems you are having. I guess it's a 15 minute fix at the dealership as long as they find the actual problem.
I'll check with my dealership on Monday, and let you know EXACTLY what they did to fix my demo car.