Tach/speedo problems on a 97 Z24

on my 97 z24 with manual trans I've noticed that when I start the car in neutral then put it in 1st gear the speedometer jumps to about 10 mph. It also does it when I have the clutch depressed or if in neutral and I rev the engine to 3000 or higher rpm. has anyone heard of this before? Another issue I've noticed is when I rev the car the tach is kinda bouncy, it always seems to be accurate, but it doesn't sweep very smoothly. But if I'm in gear and accelerating then its nice and smooth, Is this just how these cars are or do I have a problem?