Speedo Problems

I've got a 97 GT with a 5 speed and 18" wheels on it. Judging by the tach and the way it pulls, it has a stock gear in the rear. My speedometer will work sometimes and when it does it seems to be accurate. But most of the time, it doesn't work properly. It will stay at 0 mph until I reach around 35-40 mph and then will float up and when I reach around 55-60 mph it usually reads at 90 mph. Also my cruise does not work at all but I wouldn't think that the two would be related. When I bought the car, the needle for the speedo was pegged past 150 mph. I noticed that it would try to work when I would start the car so I took the guage pod out and lifted the needle over the peg. It worked properly for a while but now it mostly floats up to 90 or more depending on my speed.

What would be causing this? Is it the wheels and an improper speedo gear or maybe a faulty speedo gear?

I do know that a new transmission was installed in the car before I bought but I don't know what kind.

Thanks in advance.