I've got a 1999 pontiac grand am gt with a 3.4l engine and just under 100k miles. I've owned it since it had 6200 on it.

Several times I've seen this happen where usually it's when I'm going slow like coming to a stop, or I'm shifting to drive out of reverse. I'm going this slow speed (maybe around 5mph) and all the sudden my speedometer needle jumps up to 150mph (the limit on my car). Sometimes it sits there for a second, sometimes not that long.
The car is OBVIOUSLY not going that fast, and the engine does NOT rev up. But the car won't go if I hit the gas, it acts like it's in neutral.

I took it in (for another problem) and asked about it. The professionals told me they scanned it and said "your car has never gone that fast". I said DUH, it's not the car, it's the speedometer! I've had the car for 10 years, it's done this a handful of times. But it's creepy and I don't want it to lead to something else.

Thanks
Rita

This is definatly a problem with the instrument cluster itself. The speedos in these cars are electronic and have a problem with the circuit board. An authorized repair center is the only way to go. Personally if it doesn't get any worse I wouldn't bother, it really won't cause any problems, the speedo should remain accurate unless you notice it acting up more.