I was having a smilar problem. I have a 2004, I was driving back to New Jersey from Florida in October. In North Carolina I am doing about 70 and happened to look down and say my speedometer needle was at 25...So I played with the gas, slowing down speeding up, and it kept going down.
Next morning, started up hit the highway, worked fine. About 20 mintues later I lost the speedometer again. Rest of the ride was long. I couldn't take it to the shop yet, had no money. So the past few months it would work about 5 mintues at the beginning of the day.
The other day I was pulling out of the drive way and it said I was doing 110!!!
Put it in the shop finally today. They said the instrument cluster needed to be replaced. I have no choice I had to get it done, and I dont want to mess with electronics on my own. Costing $480!!!
I had the car almsot 3 years, and besides from replacing the breaks this is the most major headache with it. Other than that this is the best car I have ever owned!