2003 T’bird 900 Erratic Speedo

My thunderbird Speedo jumps back and forth between 60 and 80 mph constantly when travelling at a constant 70mph on motorways. It works perfectly under 70 mph.
I’ve changed the cable and it’s the same. I’m going to check the drive in the front wheel but wondered if anyone knows of someone that repairs Speedo’s or a cheaper replacement to the eye wateringly expensive ones of flea bay. thank you