mitch eire

hi everyone .. i have read through allot of posts but i cannot find anything about my issue .. i have a 1998 clk 230 and for some reason when i turn on my car and start to drive my speedo starts to read like the ticking of a clock hand . it goes all the way around on acceloration and stays there .. sometimes when i turn the car off it will "tick" all the way back to zero and will be fine for the next journey but soon it will just go all the way back around under normal driving.it seems like it is getting a signal but a bad one at that..
is there a sensor i could change to sort this out ? if so where is it ?
thanks in advance