Sometimes, the problem is not the cable but the hookup of the ZF angle-drive adapter to the transaxle. There is a double-thread nut attaching the angle drive to the transaxle that is right-hand-thread on one end and left-hand-thread on the other. The nut must be screwed all they way onto the right-angle adapter (finger tight), then positioned against the threads protruding from the gearbox. Thread the other end of the nut down onto the protruding threads, only slightly more than hand-tight. This engages a 'finger' on the ZF shaft into a fork in the right angle adapter, to the max possible depth. Inspect your right angle adapter for fork damage, then try this- it's simple.
Not doing it this way will not allow the finger-and-fork setup to engage to max depth and can cause chatter in the speedo, cable & drive. Eventually that chatter will break one side of the right angle adapter 'fork' off- which actually may be your problem. A rare-as-an-honest-politician NOS right-angle adapter is now well over $300 USD. Over the years I've fixed a couple of dozen right-angle speedo drives for owners and they're all the same: one side of the fork was beaten off. Torquing the double-thread nut down real tight also does not cure chatter nor stop oil leaks- ZF uses an internal oil seal.
Note- do NOT attempt to remove the protruding blade mechanism from inside a ZF unless you know what you're doing! The ZF 5-DS 25 transaxle was designed to run upside-down in the Ford GT-40 and Mangusta compared to our Panteras. There is a 4" long extension shaft on the other end of the speedo 'blade' (retained by an innocent looking 10mm bolt nearby). Careless removal attempts will cause the shaft to fall inside the gearbox. On a GT-40 or 'Goose with the cases upside down, the shaft & seal holder slide harmlessly out onto the ground. If this happens to you, a removal and total tear-down of your Pantera ZF will be required to retrieve the shaft. There's not much extra room inside those boxes: running the car any distance at all with a loose steel speedo shaft inside WILL crack the aluminum cases and probably break a tooth or two off a gear. Pricing a rebuilt ZF today will stop your heart....