03 lx Speedometer Jumping

[Cojacks]

Hello,

2003 ford focus lx Manual

My speedometer seems to be jumping when i'm driving the car(once in a while).The engines looses power if the speedo jumps past 150-160 ish(I think it's hitting the speed limitor). I was wondering if i need to replace the clustor or if it could be a faulty speed sensor. It has been hooked up to a scaner and found no problems; however, the problem does not happen when its idling(only driving).

If anyone else has or has had this prob please let me know

Thanks

ps- What is the speed limitor set at ?