I assume it's a 'fly by wire' speedo in which case it can only be electrical interferencefrom another bit of the engine management system, especially when you've replaced the sender. A big culprit for misbehaving electrics is a badly earthed engine so that's where I'd go next, if you've done that then try spraying WD40 or any water dispersant fluid on all the wiring you can find, as it may be power leakage somewhere else emitting a frequency wave that interferes with the sender for your speedo. I'm no expert on modern car electronics, and find they get more and more complicated as I get older, and less able to learn new tricks.
Giok
[check all fuses]