Speedo cable

[jack wingard]

On my new purchase the car had broken speedo cable. Replaced with new insert. That lasted 18 miles. I noticed the outer cable spiral was once possibly slightly kinked at both ends then tape covering the split cable. Replaced the entire cable assembly. That lasted 80 miles before it broke. All 3 times the insert was broken at the tranny end. What could be the problem?

PS when working the speedo is smooth running and not jumpy or noisy