while driving my 2000 civic dx the speedometer goes up to about 20 -30 mph and then gets stuck if starts fluctuating rapidlly but doesnot pass 30..it's not physically getting stuck so don't know if its the sensor or what.. anyone have a an idea?? Thanks for the help..

 

If the odometer keeps rolling normally and accurately at higher speeds, the sensor is OK.

Sounds like the speedometer unit itself or something is blocking the needle from going up.