Years ago I had the same problem with my plus 8 with an almost identical setup. I contacted both speedycables and speedograph. Both said they could help but I had to send my cables to them to act as templates. I was idly fiddling with the road speed sensor when I noticed it was very stiff. So I managed to open it up and found it full of semi solid grease. After cleaning and reassembly the flickering needle problem disappeared. Opening the sensor required drilling out the rivet heads which was easy enough but reassembly required the rivet shanks to be drilled and tapped for set screws
