a Very common fault!!
 

I know all 124's seem to have problems with cruise controll but mine will work but it speeds up then taps off then puts full throttle then taps off....you get the picture but all my friends think its hilarious, I take that its reading incorrect speeds? What part needs repairing or replacing?

Secondly earlier I posted a a thread but I can't find it but it was about what I thought was low voltage becuause instrument lights and cubby hole lights etc.. are all dimmer than other 124's but the voltage reads 12.something when off and 13.68V when running, even across the battery so its then exactly right?

I know on a 123 that is a spedo cable, my wifes car needs one. At low speeds the spedo fluctuates wildly. While its not as noticable at higher speeds the cruise control computer can 'see' it. I don't remember if the 124 has a physical cable or if its an electrical signal.

I can answer that one. The 124s had a speedo cable. I have had to mess with it many times to remove the instrument cluster. At least my 87 does, not sure if it was changed in the late ones.

I thought it was a speedo cable but wasn't sure how the cruise control read the speed, I know it was replaced not so long ago but yes it fluctuates alot at low speeds and a little even at higher so I will get that replaced first of all. Could the actuall spedometer be tired?