Dont worry.....tuner here. Well first lets start with basic electrical laws of conductivity. If it is not grounded you get an electrical spike that will not let the gauge operate at all. If it is the speed sensor when you start driving it will be fine until there is a significant electrical draw on the line(about 3000rpms)and will jump wildly around. As the other guy said his would be fine then later jump means he has a really good line run and it is just not overloading the tach. Tachometers are callibrated to be sensitive to draw as that is how they translate an electrical signal into rpms. You can do two things to fix this: 1. run the tach wire to the service connection on a 96-00(blue wire going to clip with cover on drivers side in engine bay near strut tower towards winshield)and connect rpm wire there(safest method)

2. run the wire to the negative side of the ignition coil(location varies among years call honda service dept and they will tell you just tell them you originally bought the car there and they will be more than happy to help or just get a chiltons manual.

That will solve your jumping tach prob you wouldnt beleive how many people have this problem.........there is your solution.