We do not have a speedo cable its all electronic.
I have the same problem as you do, my speedometer does not work, but then sometimes it does. Majority of the times it doesnt. Usually if I go over a bump or hit the cluster it'll work, same as you.
Unfortunately I havent read or seen any way to fix this problem, so I've just let it be. I usually just read the tach as my speedometer or not even worry about it. More then likely youll have to buy a new cluster, ebay is your friend