Cruise control erratic

I have a 90 300d.
My cruise control is being very erratic. I will click set, and then it accelerates probably at least 5 mph faster than i set it and then it shuts it off and then back on, and then back off and on etc. Has any body had this happened? I am lost at where to start.

I have a alldata manual on it but its not much help. Is there some sort of adjustment with it?

Thanks
Jerritt