I spent a couple of hours going nuts until I finally figured it out.
I have a F77 that arrived a few days ago. I put some keys on it and gave it a spin. Awful, simply awful.
Key presses would register or not register. Random text would appear while typing. Unusable.

Turns out I was using a 4 port non-powered Anker USB hub. Works fine for my model M’s, and the rest of my keyboards BUT not the F77.

I then plugged the F77 into a powered USB hub and tada, all is good.

Hope this helps others.