Jumpy Speedometer

Hello-
I have a 91 miata, here recently the speedometer has been sticking at zero then quickly jumping to twenty and getting stuck and so on, I dont think it ever tells me the correct speed im going considering i have to go 70 every day but it get stuck permanatly at 65. I took it apart and sprayed WD-40 in the cable area and noticed a think build of a brown oily substance. . .possiably the past owner had the same problem. . ne way the speedometer still jumps, how can i tell if it is the cable or the speedometer it self causing the problem. Who knows the WD-40 may have it loose by morning. . . .

if it sticks then jumps quickly to a higher speed then it is usually in the speedo head itself... just a steady fluttering of the needle is usually the cable

any chance you might know where i can get a NEW unit or refurbished unit other than ebay?

you might try your luck with a junkyard unit.... its a risk, but its lots cheaper than a new one