My cruise control on my w124 is dodgy, it puts full throttle then nothing continuously, its very uncomfortable. Is the OVP relay the box under the steering wheel?, I had someone with a few spare ones to test and they were not the right ones, all they did was make the car put full throttle solidly, it was pretty funny/dangerous so I put mine back. Is there something else that could be giving this problem, I know an old speedo cable could be an issue too.